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distant-traveller:

A star set to explode


Floating at the centre of this new Hubble image is a lidless purple eye, staring back at us through space. This ethereal object, known officially as [SBW2007] 1 but sometimes nicknamed SBW1, is a nebula with a giant star at its centre. The star was originally twenty times more massive than our Sun, and is now encased in a swirling ring of purple gas, the remains of the distant era when it cast off its outer layers via violent pulsations and winds.



But the star is not just any star; scientists say that it is destined to go supernova! 26 years ago, another star with striking similarities went supernova — SN 1987A. Early Hubble images of SN 1987A show eerie similarities to SBW1. Both stars had identical rings of the same size and age, which were travelling at similar speeds; both were located in similar HII regions; and they had the same brightness. In this way SBW1 is a snapshot of SN1987a’s appearance before it exploded, and unsurprisingly, astronomers love studying them together.
At a distance of more than 20 000 light-years it will be safe to watch when the supernova goes off. If we are very lucky it may happen in our own lifetimes…

Image credit: ESA/Hubble & NASA; acknowledgement: Nick Rose

distant-traveller:

A star set to explode

Floating at the centre of this new Hubble image is a lidless purple eye, staring back at us through space. This ethereal object, known officially as [SBW2007] 1 but sometimes nicknamed SBW1, is a nebula with a giant star at its centre. The star was originally twenty times more massive than our Sun, and is now encased in a swirling ring of purple gas, the remains of the distant era when it cast off its outer layers via violent pulsations and winds.

But the star is not just any star; scientists say that it is destined to go supernova! 26 years ago, another star with striking similarities went supernova — SN 1987A. Early Hubble images of SN 1987A show eerie similarities to SBW1. Both stars had identical rings of the same size and age, which were travelling at similar speeds; both were located in similar HII regions; and they had the same brightness. In this way SBW1 is a snapshot of SN1987a’s appearance before it exploded, and unsurprisingly, astronomers love studying them together.

At a distance of more than 20 000 light-years it will be safe to watch when the supernova goes off. If we are very lucky it may happen in our own lifetimes…

Image credit: ESA/Hubble & NASA; acknowledgement: Nick Rose

, #SBW2007 #space #astronomyfacts #astrophysics #star #supernova

mucholderthen:

THE SOLAR NEBULA MODEL for the Formation of Planetary Systems

According to the nebular hypothesis, the Sun and planets formed at the same time during the collapse of an interstellar cloud of gas and dust.  It is thought that this is how most stars and their planetary systems formed.

  1. A sun and planetary system form from a gravitationally collapsing ball of gas and dust that flattens to form a spinning disk. The disk reaches an equilibrium size while the protosun continues to collapse in the center.  [Source: Astro - U of Nebraska]
  2. [Source Astro 001h PSU]
  3. Temperature decreases with radial distance from the proto-sun  [Source: Astrophysical & Planetary Sciences, University of Colorado]
  4. Unformed planets circle the new born Sun, before its nuclear fires burst forth. (Courtesy of Helmut K. Wimmer, Hayden Planetarium, American Museum of Natural History.)  (via NASA’s Cosmos)
  5. A sun-like star as it might have looked at 1 million years of age, as minerals condense from the primordial gas cloud that will eventually form a sun and planets. Credit: NASA/JPL-Caltech/T. Pyle, SSC (via Phys.org)
  6. A narrow asteroid belt filled with rocks and dusty debris orbits a star similar to our own Sun at 30 million years of age. [NASA/JPL-Caltech] (via Noble Gas Cosmochemistry Group)

, #astronomyfacts #astronomy #solar nebula #solar system formation #early solar system #solar system #planet #planets #star #stars #solar nebula model #interstellar cloud

kenobi-wan-obi:

14 years of Hubble Observations Presents Unprecedented Views of Supersonic Jets From Young Stars Shedding Light on How Stars Like Our Sun Are Formed

Using data observations NASA’s Hubble space telescope, Astronomers were able to create streaming movies of the supersonic jets that release energy from young stars to possibly form into stars like our star, the sun.

Stars aren’t shy about sending out birth announcements. They fire off energetic jets of glowing gas travelling at supersonic speeds in opposite directions through space.

Although astronomers have looked at still pictures of stellar jets for decades, now they can watch movies, thanks to the NASA/ESA Hubble Space Telescope.

An international team of scientists led by astronomer Patrick Hartigan of Rice University in Houston, USA, has collected enough high-resolution Hubble images over a 14-year period to stitch together time-lapse movies of young jets ejected from three stars.

The moving pictures offer a unique view of stellar phenomena that move and change over just a few years. Most astronomical processes change over timescales that are much longer than a human lifetime.

The movies reveal the motion of the speedy outflows as they tear through the interstellar environments. Never-before-seen details in the jets’ structure include knots of gas brightening and dimming and collisions between fast-moving and slow-moving material, creating glowing arrowhead features. These phenomena are providing clues about the final stages of a star’s birth, offering a peek at how the Sun behaved 4.5 billion years ago.

“For the first time we can actually observe how these jets interact with their surroundings by watching these time-lapse movies,” said Hartigan. “Those interactions tell us how young stars influence the environments out of which they form. With movies like these, we can now compare observations of jets with those produced by computer simulations and laboratory experiments to see which aspects of the interactions we understand and which we don’t understand.”

Hartigan’s team’s results appear in the 20 July 2011 issue of the Astrophysical Journal.

Movie/Data Source: Heic1113 Science Release

, #gif #gifs #space #Hubble #NASA #astronomyfacts #astrophysics #star #stars #stellar formation #supersonic jet
thenewenlightenmentage:

What is a Main Sequence Star?

Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores. Most of the stars in the universe — about 90 percent of them — are main sequence stars. The sun is a main sequence star. These stars can range from about a tenth of the mass of the sun to up to 200 times as massive.1

What properties do these stars have?  It would be best to understand their properties in light of the physical processes occurring in their interiors.

First is the hydrostatic balance, also called hydrostatic equilibrium. This determines the densitystructure of the star as the internal pressure gradient balances against the force of gravity. Another way of thinking about this is to imagine the star as a large number of nested thin spherical shells, resembling the structure of an onion. The inward forces on each shell consist of the gravitational pull from all the shells inside it, and the gas and radiation pressure on the outside of the shell. The only outward force on each shell is the gas and radiation pressure on the inside of the shell. There is no gravitational force from material outside the shell (in physics, this is known as Gauss’s law) In hydrostatic equilibrium, the inward and outward forces must balance. If they don’t, the shell will either collapse or expand. The timescale for this to occur is called the “free-fall timescale,” and it is about 2,000 seconds for a star like the Sun. Since we know the Sun has been essentially stable over the age of Earth (several billion years), the hydrostatic balance must be maintained to a very high level of accuracy. A consequence of hydrostatic balance is that the pressure on each shell from material outside it must be less than the pressure from material inside it. This is because gravity acts only in the inward direction. Thus, the pressure in the star must decrease with increasing radius. This is an intuitively obvious result, as the pressure at the center of the star is greater than it is at the surface.
The second physical process to consider is the transport of energy from the interior of the star to the surface. The interior of the star is heated by nuclear reactions, while at the surface electromagnetic radiation can escape essentially freely into space. This situation is analogous to a pot of water on a stove, in which heat is deposited at the bottom by the stove burner, and is transported upward through the water to the surface where it can escape. The rate at which the water on the stove can transport the heat determines the temperature. A lid on the pot will cause the temperature in the water to be higher than it would be with no lid, since heat is impeded from escaping the pot. In the case of a star, the temperature of the gas determines the density structure via the hydrostatic equilibrium condition, so understanding the transport mechanism is important.
The transport can occur by either of two mechanisms: either the energy is carried by radiation, or it is carried by convection. Radiation is the mechanism by which Earth receives heat from the Sun, and its efficiency depends on the opacity of the material that the radiation must traverse. Opacity is a measure of the transparency of a gas, and it depends on the gas temperature, density and elemental composition. Convection is analogous to the turbulent motion in a pot of water as it boils. It involves motion of the fluid in the pot (or the interior of the star) which transports heat. The operation of convection depends on how easily the gas can move, i.e., its viscosity, and any forces that tend to resist the convective motion, such as gravity. In addition, convection can only operate if it transports more heat than radiation. This is important. When the opacity is high and radiation is inefficient, convection takes over. The details of the efficiency of convection are not well understood, and they are probably the major source of uncertainty in the study of stellar structure and evolution. A third energy transport mechanism, conduction, is relatively unimportant in stellar interiors.
Main sequence stars have zones (in radius) that are convective, and zones that are radiative, and the location of these zones depends on the behavior of the opacity, in addition to the other properties of the star. Massive stars, which are those greater than several solar masses, are convective deep in their cores, and are radiative in their outer layers. Low mass stars, which have a mass comparable to or less than the Sun, are convective in their outer layers and radiative in their cores. Intermediate mass stars (spectral type A) may be radiative throughout. Convection is likely to be important in determining other properties of the star. The existence of a hot corona may be associated with active convection in the outer layers, and the depth of the convective layer determines the extent to which material from the deep interior of the star is mixed into the outer layers. Since interior material is likely to have undergone nuclear reactions, which change the elemental abundances, this mixing affects the abundances in the star’s atmosphere. These can be observed by studying stellar spectra. They may also be ejected from the star in a stellar wind, and so affect the composition of interstellar gas.
The final ingredient in determining the structure of a main sequence star is the source of heat in the interior: nuclear reactions. There are many of these events, but there is still some uncertainty about the exact rate of reactions. This is because the fundamental particles produced by nuclear reactions, called solar neutrinos, react weakly with other particles. Most pass right through the planet, making them extremely difficult to detect.
The basic reactions that operate on the main sequence are fusion reactions, which convert hydrogen nuclei (protons) into helium nuclei. These reactions require high temperatures (greater than 10 million degrees Celsius and densities (greater than a few hundred grams per cubic centimeter), and the rates are sensitive functions of temperature and density. This is the factor that ultimately determines the lifetime of a main sequence star. More massive stars have greater central temperatures and densities, and exhaust their nuclear fuel more rapidly (in spite of the fact that they have more of it) than do lower-mass stars. It turns out that the main sequence lifetime is a sensitive function of mass. For a star like the Sun, the main-sequence stage lasts about 10 billion years, whereas a star 10 times as massive will be 1,000 to 10,000 times as bright but will only last about 20 million years. A star with 1/10 the Sun’s mass may only be 1/1,000 to 1/10,000 of its brightness, but will last about 1 trillion years.
It is interesting to consider what would happen to the star if the nuclear reactions were to turn off suddenly. The timescale required for the energy from a photon released at the center of the star to make its way to the surface is approximately 1 million years for a star like the Sun. Along the way, the original gamma-ray photon interacts with the gas in the star and loses energy. Through multiple interactions like this, this energy “random walks” its way out of the star, ultimately being emitted at the surface as many photons in the ultraviolet and visual wavelengths. So, if the nuclear reactions were to turn off today, the Sun’s luminosity would stay approximately constant for a long time by human standards. We do have historical records that tell us that the Sun’s output has been approximately constant over the course of written human history, so scientists are fairly confident that the nuclear reactions are still operating. However, there is the possibility that nuclear energy generation in the center of the Sun is not perfectly constant.
The three physical processes discussed so far — hydrostatic equilibrium, radiation transport, and nuclear energy generation — serve to determine the structure of a star. But the devil is in the details. The areas of greatest uncertainty are the behavior of opacity and convection, and these are active areas of scientific research.
A convenient way to characterize a star from observations is by its luminosity, as well as its color, or temperature. It is customary to plot these two quantities in an x-y plot, called a Hertzsprung-Russell diagram. It turns out that when this is done for main sequence stars with a range of masses, the points tend to occupy a narrow band in the diagram. The location of a main sequence star in the diagram depends only on its mass (see Figure below).
2

The Sun is currently a main sequence star.  When it can no longer fuse hydrogen into helium, it will be because the Sun has lost its equilibrium.  Gravity will then begin to essentially crush the Sun.  At this point, the Sun will fuse helium into carbon at its center.  Its outer layers, in an effort to conserve energy, will expand.  The Sun will then be cooler but will appear more luminous.  This is called the red giant phase.  The Sun will, unfortunately, continue to lose its outer layers; it will also fuse carbon into heavier elements like oxygen, nitrogen, silicon and iron.  It will eventually collapse into a very dense white dwarf and after millions of years, it will cease to emit heat and light—and thus, become a black dwarf.
1 http://www.space.com/22437-main-sequence-stars.html
2 http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html
GIF Credit: Watch Here

thenewenlightenmentage:

What is a Main Sequence Star?

Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores. Most of the stars in the universe — about 90 percent of them — are main sequence stars. The sun is a main sequence star. These stars can range from about a tenth of the mass of the sun to up to 200 times as massive.1

What properties do these stars have?  It would be best to understand their properties in light of the physical processes occurring in their interiors.

First is the hydrostatic balance, also called hydrostatic equilibrium. This determines the densitystructure of the star as the internal pressure gradient balances against the force of gravity. Another way of thinking about this is to imagine the star as a large number of nested thin spherical shells, resembling the structure of an onion. The inward forces on each shell consist of the gravitational pull from all the shells inside it, and the gas and radiation pressure on the outside of the shell. The only outward force on each shell is the gas and radiation pressure on the inside of the shell. There is no gravitational force from material outside the shell (in physics, this is known as Gauss’s law) In hydrostatic equilibrium, the inward and outward forces must balance. If they don’t, the shell will either collapse or expand. The timescale for this to occur is called the “free-fall timescale,” and it is about 2,000 seconds for a star like the Sun. Since we know the Sun has been essentially stable over the age of Earth (several billion years), the hydrostatic balance must be maintained to a very high level of accuracy. A consequence of hydrostatic balance is that the pressure on each shell from material outside it must be less than the pressure from material inside it. This is because gravity acts only in the inward direction. Thus, the pressure in the star must decrease with increasing radius. This is an intuitively obvious result, as the pressure at the center of the star is greater than it is at the surface.

The second physical process to consider is the transport of energy from the interior of the star to the surface. The interior of the star is heated by nuclear reactions, while at the surface electromagnetic radiation can escape essentially freely into space. This situation is analogous to a pot of water on a stove, in which heat is deposited at the bottom by the stove burner, and is transported upward through the water to the surface where it can escape. The rate at which the water on the stove can transport the heat determines the temperature. A lid on the pot will cause the temperature in the water to be higher than it would be with no lid, since heat is impeded from escaping the pot. In the case of a star, the temperature of the gas determines the density structure via the hydrostatic equilibrium condition, so understanding the transport mechanism is important.

The transport can occur by either of two mechanisms: either the energy is carried by radiation, or it is carried by convection. Radiation is the mechanism by which Earth receives heat from the Sun, and its efficiency depends on the opacity of the material that the radiation must traverse. Opacity is a measure of the transparency of a gas, and it depends on the gas temperature, density and elemental composition. Convection is analogous to the turbulent motion in a pot of water as it boils. It involves motion of the fluid in the pot (or the interior of the star) which transports heat. The operation of convection depends on how easily the gas can move, i.e., its viscosity, and any forces that tend to resist the convective motion, such as gravity. In addition, convection can only operate if it transports more heat than radiation. This is important. When the opacity is high and radiation is inefficient, convection takes over. The details of the efficiency of convection are not well understood, and they are probably the major source of uncertainty in the study of stellar structure and evolution. A third energy transport mechanism, conduction, is relatively unimportant in stellar interiors.

Main sequence stars have zones (in radius) that are convective, and zones that are radiative, and the location of these zones depends on the behavior of the opacity, in addition to the other properties of the star. Massive stars, which are those greater than several solar masses, are convective deep in their cores, and are radiative in their outer layers. Low mass stars, which have a mass comparable to or less than the Sun, are convective in their outer layers and radiative in their cores. Intermediate mass stars (spectral type A) may be radiative throughout. Convection is likely to be important in determining other properties of the star. The existence of a hot corona may be associated with active convection in the outer layers, and the depth of the convective layer determines the extent to which material from the deep interior of the star is mixed into the outer layers. Since interior material is likely to have undergone nuclear reactions, which change the elemental abundances, this mixing affects the abundances in the star’s atmosphere. These can be observed by studying stellar spectra. They may also be ejected from the star in a stellar wind, and so affect the composition of interstellar gas.

The final ingredient in determining the structure of a main sequence star is the source of heat in the interior: nuclear reactions. There are many of these events, but there is still some uncertainty about the exact rate of reactions. This is because the fundamental particles produced by nuclear reactions, called solar neutrinos, react weakly with other particles. Most pass right through the planet, making them extremely difficult to detect.

The basic reactions that operate on the main sequence are fusion reactions, which convert hydrogen nuclei (protons) into helium nuclei. These reactions require high temperatures (greater than 10 million degrees Celsius and densities (greater than a few hundred grams per cubic centimeter), and the rates are sensitive functions of temperature and density. This is the factor that ultimately determines the lifetime of a main sequence star. More massive stars have greater central temperatures and densities, and exhaust their nuclear fuel more rapidly (in spite of the fact that they have more of it) than do lower-mass stars. It turns out that the main sequence lifetime is a sensitive function of mass. For a star like the Sun, the main-sequence stage lasts about 10 billion years, whereas a star 10 times as massive will be 1,000 to 10,000 times as bright but will only last about 20 million years. A star with 1/10 the Sun’s mass may only be 1/1,000 to 1/10,000 of its brightness, but will last about 1 trillion years.

It is interesting to consider what would happen to the star if the nuclear reactions were to turn off suddenly. The timescale required for the energy from a photon released at the center of the star to make its way to the surface is approximately 1 million years for a star like the Sun. Along the way, the original gamma-ray photon interacts with the gas in the star and loses energy. Through multiple interactions like this, this energy “random walks” its way out of the star, ultimately being emitted at the surface as many photons in the ultraviolet and visual wavelengths. So, if the nuclear reactions were to turn off today, the Sun’s luminosity would stay approximately constant for a long time by human standards. We do have historical records that tell us that the Sun’s output has been approximately constant over the course of written human history, so scientists are fairly confident that the nuclear reactions are still operating. However, there is the possibility that nuclear energy generation in the center of the Sun is not perfectly constant.

The three physical processes discussed so far — hydrostatic equilibrium, radiation transport, and nuclear energy generation — serve to determine the structure of a star. But the devil is in the details. The areas of greatest uncertainty are the behavior of opacity and convection, and these are active areas of scientific research.

A convenient way to characterize a star from observations is by its luminosity, as well as its color, or temperature. It is customary to plot these two quantities in an x-y plot, called a Hertzsprung-Russell diagram. It turns out that when this is done for main sequence stars with a range of masses, the points tend to occupy a narrow band in the diagram. The location of a main sequence star in the diagram depends only on its mass (see Figure below).

2

The Sun is currently a main sequence star.  When it can no longer fuse hydrogen into helium, it will be because the Sun has lost its equilibrium.  Gravity will then begin to essentially crush the Sun.  At this point, the Sun will fuse helium into carbon at its center.  Its outer layers, in an effort to conserve energy, will expand.  The Sun will then be cooler but will appear more luminous.  This is called the red giant phase.  The Sun will, unfortunately, continue to lose its outer layers; it will also fuse carbon into heavier elements like oxygen, nitrogen, silicon and iron.  It will eventually collapse into a very dense white dwarf and after millions of years, it will cease to emit heat and light—and thus, become a black dwarf.

http://www.space.com/22437-main-sequence-stars.html

http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html

GIF Credit: Watch Here

, #astronomyfacts #star #black dwarf #white dwarf #main sequence star
kenobi-wan-obi:


Newborn Stars Shoot Cosmic Jets

Dozens of newborn stars sprouting jets from their dusty cocoons have been spotted in images from NASA’s Spitzer Space Telescope. In this view showing a portion of sky near Canis Major, infrared data from Spitzer are green and blue, while longer-wavelength infrared light from NASA’s Wide-field Infrared Survey Explorer (WISE) are red.
The jets appear in green, while young stars are a yellow-orange hue. Some of the jets can be seen as streaks, while others appear as blobs because only portions of the jet can be seen. In some cases, the stars producing jets can’t be seen while their jets can. Those stars are so embedded in their dusty cocoon that they are too faint to be seen at Spitzer’s wavelengths.
This is a lesser-known region of star formation, located near the outer edge of our Milky Way galaxy. Spitzer is showing that even these more sparse regions of the galaxy are aglow with stellar youth.

kenobi-wan-obi:

Newborn Stars Shoot Cosmic Jets

Dozens of newborn stars sprouting jets from their dusty cocoons have been spotted in images from NASA’s Spitzer Space Telescope. In this view showing a portion of sky near Canis Major, infrared data from Spitzer are green and blue, while longer-wavelength infrared light from NASA’s Wide-field Infrared Survey Explorer (WISE) are red.

The jets appear in green, while young stars are a yellow-orange hue. Some of the jets can be seen as streaks, while others appear as blobs because only portions of the jet can be seen. In some cases, the stars producing jets can’t be seen while their jets can. Those stars are so embedded in their dusty cocoon that they are too faint to be seen at Spitzer’s wavelengths.

This is a lesser-known region of star formation, located near the outer edge of our Milky Way galaxy. Spitzer is showing that even these more sparse regions of the galaxy are aglow with stellar youth.

, #science #astronomyfacts #star
A Beautiful End to a Star’s Life via NASA
Composite image of planetary nebula NGC 2392 - Image: X-ray: NASA/CXC/IAA-CSIC/N. Ruiz et al; Optical: NASA/STScI

Stars like the Sun can become remarkably photogenic at the end of their life. A good example is NGC 2392, which is located about 4,200 light years from Earth. NGC 2392, nicknamed the “Eskimo Nebula,” is what astronomers call a planetary nebula. This designation, however, is deceiving because planetary nebulas actually have nothing to do with planets. The term is simply a historic relic since these objects looked like planetary disks to astronomers in earlier times looking through small optical telescopes.
Instead, planetary nebulas form when a star uses up all of the hydrogen in its core – an event our Sun will go through in about five billion years. When this happens, the star begins to cool and expand, increasing its radius by tens to hundreds of times its original size. Eventually, the outer layers of the star are carried away by a thick 50,000 kilometer per hour wind, leaving behind a hot core. This hot core has a surface temperature of about 50,000 C, and is ejecting its outer layers in a much faster wind traveling six million kilometers per hour. The radiation from the hot star and the interaction of its fast wind with the slower wind creates the complex and filamentary shell of a planetary nebula. Eventually the remnant star will collapse to form a white dwarf star.
Now days, astronomers using space-based telescopes are able to observe planetary nebulas such as NGC 2392 in ways their scientific ancestors probably could never imagine. This composite image of NGC 2392 contains X-ray data fromNASA's Chandra X-ray Observatory in purple showing the location of million-degree gas near the center of the planetary nebula. Data from the Hubble Space Telescope show – colored red, green and blue – the intricate pattern of the outer layers of the star that have been ejected. The comet-shaped filaments form when the faster wind and radiation from the central star interact with cooler shells of dust and gas that were already ejected by the star.
The observations of NGC 2392 were part of a study of three planetary nebulas with hot gas in their center. The Chandra data show that NGC 2392 has unusually high levels of X-ray emission compared to the other two. This leads researchers to deduce that there is an unseen companion to the hot central star in NGC 2392. The interaction between a pair of binary stars could explain the elevated X-ray emission found there. Meanwhile, the fainter X-ray emission observed in the two other planetary nebulas in the sample – IC 418 and NGC 6826 – is likely produced by shock fronts (like sonic booms) in the wind from the central star.
A paper describing these results is available online here and was published in the April 10th, 2013 issue of The Astrophysical Journal. The first author is Nieves Ruiz of the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Granada, Spain, and the other authors are You-Hua Chu, and Robert Gruendl from the University of Illinois, Urbana; Martín Guerrero from the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Granada, Spain, and Ralf Jacob, Detlef Schönberner and Matthias Steffen from the Leibniz-Institut Für Astrophysik in Potsdam (AIP), Germany.

Learn more about planetary nebulae here.
Check out Chandra’s Flickr photoset here.

A Beautiful End to a Star’s Life via NASA

Composite image of planetary nebula NGC 2392 - Image: X-ray: NASA/CXC/IAA-CSIC/N. Ruiz et al; Optical: NASA/STScI

Stars like the Sun can become remarkably photogenic at the end of their life. A good example is NGC 2392, which is located about 4,200 light years from Earth. NGC 2392, nicknamed the “Eskimo Nebula,” is what astronomers call a planetary nebula. This designation, however, is deceiving because planetary nebulas actually have nothing to do with planets. The term is simply a historic relic since these objects looked like planetary disks to astronomers in earlier times looking through small optical telescopes.

Instead, planetary nebulas form when a star uses up all of the hydrogen in its core – an event our Sun will go through in about five billion years. When this happens, the star begins to cool and expand, increasing its radius by tens to hundreds of times its original size. Eventually, the outer layers of the star are carried away by a thick 50,000 kilometer per hour wind, leaving behind a hot core. This hot core has a surface temperature of about 50,000 C, and is ejecting its outer layers in a much faster wind traveling six million kilometers per hour. The radiation from the hot star and the interaction of its fast wind with the slower wind creates the complex and filamentary shell of a planetary nebula. Eventually the remnant star will collapse to form a white dwarf star.

Now days, astronomers using space-based telescopes are able to observe planetary nebulas such as NGC 2392 in ways their scientific ancestors probably could never imagine. This composite image of NGC 2392 contains X-ray data fromNASA's Chandra X-ray Observatory in purple showing the location of million-degree gas near the center of the planetary nebula. Data from the Hubble Space Telescope show – colored red, green and blue – the intricate pattern of the outer layers of the star that have been ejected. The comet-shaped filaments form when the faster wind and radiation from the central star interact with cooler shells of dust and gas that were already ejected by the star.

The observations of NGC 2392 were part of a study of three planetary nebulas with hot gas in their center. The Chandra data show that NGC 2392 has unusually high levels of X-ray emission compared to the other two. This leads researchers to deduce that there is an unseen companion to the hot central star in NGC 2392. The interaction between a pair of binary stars could explain the elevated X-ray emission found there. Meanwhile, the fainter X-ray emission observed in the two other planetary nebulas in the sample – IC 418 and NGC 6826 – is likely produced by shock fronts (like sonic booms) in the wind from the central star.

A paper describing these results is available online here and was published in the April 10th, 2013 issue of The Astrophysical Journal. The first author is Nieves Ruiz of the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Granada, Spain, and the other authors are You-Hua Chu, and Robert Gruendl from the University of Illinois, Urbana; Martín Guerrero from the Instituto de Astrofísica de Andalucía (IAA-CSIC) in Granada, Spain, and Ralf Jacob, Detlef Schönberner and Matthias Steffen from the Leibniz-Institut Für Astrophysik in Potsdam (AIP), Germany.

Learn more about planetary nebulae here.

Check out Chandra’s Flickr photoset here.

, #astronomy #astronomyfacts #science #star #ngc 2392 #planetary nebula

atomstargazer:

What is Magnetar?

A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays. The theory regarding these objects was proposed by Robert Duncan and Christopher Thompson in 1992, but the first recorded burst of gamma rays thought to have been from a magnetar was detected on March 5, 1979. During the following decade, the magnetar hypothesis has become widely accepted as a likely explanation for soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs).

Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a greater mass than the Sun. The density of the interior of a magnetar is such that a thimble full of its substance would have a mass of over 100 million tons. Magnetars are differentiated from other neutron stars by having even stronger magnetic fields, and rotating comparatively slowly, with most magnetars completing a rotation once every one to ten seconds, compared to less than one second for a typical neutron star. This magnetic field gives rise to very strong and characteristic bursts of X-rays and gamma rays. The active life of a magnetar is short. Their strong magnetic fields decay after about 10,000 years, after which activity and strong X-ray emission cease. Given the number of magnetars observable today, one estimate puts the number of inactive magnetars in the Milky Way at 30 million or more.

Starquakes triggered on the surface of the magnetar disturb the magnetic field which encompasses it, often leading to extremely powerful gamma ray flare emissions which have been recorded on Earth in 1979, 1998, and 2004.

Magnetars, the Most Magnetic Stars In the Universe

“We only know of about 10 magnetars in the Milky Way galaxy.” remarked Dr. Peter Woods of the Universities Space Research Association. “If the antics of the magnetar we are studying now are typical, then there very well could be hundreds more out there.” NASA research has suggested there may be far more magnetars than previously thought.

Observing the explosions from these celestial bodies has been tricky. The answer lies in the timing. So how do the researchers observe what has never been seen? Leave it to NASA to develop the perfect piece of equipment to handle the job.

The Rossi X-ray Timing Explorer (RXTE), launched in December 1995 from Kennedy Space Center, Fla., was designed to observe fast-moving neutron stars, X-ray pulsars and bursts of X-rays that brighten the sky and disappear.

Some pulsars spin over a thousand times a second. A neutron star generates a gravitational pull so powerful that a marshmallow impacting the star’s surface would hit with the force of a thousand hydrogen bombs.

Magnetars, the most magnetic stars known, aren’t powered by a conventional mechanism such as nuclear fusion or rotation, according to Dr. Vicky Kaspi. “Magnetars represent a new way for a star to shine, which makes this a fascinating field,” said Kaspi.

Although not totally understood yet, magnetars have magnetic fields a thousand times stronger than ordinary neutron stars that measure a million billion Gauss, or about a hundred-trillion refrigerator magnets. For comparison, the Sun’s magnetic field is only about 5 Gauss.

Image 1 | Artist’s conception of a magnetar, with magnetic field lines

Image 2 | Magnetar SGR 1900+14 is in the exact center of the image, which shows a surrounding ring of gas seven light-years across in infrared light, as seen by the Spitzer Space Telescope. The magnetar itself is not visible at this wavelength, but it has been seen in X-ray light.

Image 3 | On 27 December 2004, a burst of gamma rays arrived into the Solar System from SGR 1806-20 (artist’s conception shown). The burst was so powerful that it had effects on Earth’s atmosphere, at a range of about 50,000 light years.

, #magnetar #Gamma Ray Burst #star #mypost #universe #Astronomy #astronomyfacts

electricspacekoolaid:

Unusual Binary Neutron Stars With Gravity 300 Billion Times the Earth

An exotic pair of binary stars have proved that Albert Einstein’s theory of relativity is still right, even in the most extreme conditions tested yet.  ”The unusual pair of stars is quite interesting in its own right but we’ve learned it is also a unique laboratory for testing the limits of one of our most fundamental physical theories, general relativity” says University of Toronto astronomy professor Marten van Kerkwijk, a member of the research team.

What makes the pair of stars exceptional are the unique characteristics of each and their close proximity to each other. One is a tiny but unusually heavyneutron star– one of the most massive confirmed to date. NamedPSRJ0348+0432, it is the remnant of a supernova explosion, and is twice as heavy as the Sun yet is only 20 kilometres across. The neutron star is a pulsar that gives off radio waves that can be picked up on Earth by radio telescopes.

The gravity at its surface is more than 300 billion times stronger than that on Earth and at its centre every sugarcube-sized volume has more than one billion tonnes of matter squeezed into it, roughly the mass of every human past and present.

The massive star spins 25 times each second and is orbited by a rather lightweight dwarf star every two and a half hours, an unusually short period. Only slightly less exotic, the white dwarf is the glowing remains of a much lighter star that has lost its envelope and is slowly cooling. It can be observed in visible light, though only with large telescopes – it is about a million times too faint to be visible with the naked eye.

In the new work, led by Bonn PhD student John Antoniadis, very precise timing of the pulsar’s spin-modulated emission with radio telescopes was used to discover a tiny but significant change in the orbital period of the binary, of eight-millionths of a second per year. Given the masses of the pulsar and the white dwarf, inferred with the help of observations of the light emitted by the white dwarf – using techniques perfected by Antoniadis and van Kerkwijk – this turns out to match exactly what Einstein’s theory predicts.

Einstein’s general theory of relativityexplains gravity as a consequence of the curvature of spacetime created by the presence of mass and energy. As two stars orbit each other, gravitational waves are emitted – wrinkles moving out in spacetime. As a result, the binary slowly loses energy, the stars move closer, and the orbital period shortens.

The test posed by PSR J0348+0432 is particularly interesting because the massive star is a truly extreme object in terms of gravity, even compared to other pulsars that have been used to test general relativity. As a result, it causes exceptionally strong distortion of spacetime. In many alternatives to Einstein’s theory, this would cause the orbit to lose energy much faster than is observed.

“The observations disprove these alternatives,” says van Kerkwijk, “and thus give further confidence that Einstein’s theory is a good description of nature – even though we know it is not a complete one, given the unresolved inconsistencies with quantum mechanics.”

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, #space #stars #star #Binary Stars #astronomy #Universe #tech #astrophysics #Einstein #physics #gravity #Albert Einstein #gravity waves #time #astronomyfacts #science
atomstargazer:


Astronomical Distances and Magnitudes


Measuring Astronomical distancesIn the words of Douglas Adams, the author of The Hitch-Hiker’s Guide to the Galaxy:  Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space. The distances involved in the universe are so vast that metres or kilometers will just not suffice. We must introduce new length scales with which can span the heavens.THE ASTRONOMICAL UNIT A.U. One natural and practical unit we can devise is the distance from the Sun to the Earth. This is the A.U. or Astronomical Unit. 1 Astronomical Unit = 149 598 000 kmLIGHT YEAR Moving to larger distances even the AU becomes unweildly and so the next suitable unit is the light-year. The light-year, as its name would suggest, is the distance travelled by light in one year. All electromagnetic waves travel at a speed of x 299,792,458 ms-1 in a vacuum and with an average year being 365.25 days, one light year is 299,792,458 x 108ms-1 x (365.25 x 24 x 60 x 60) s = 9.46073 x 1015 m. or 9.46073 x 1012 km. 1 lt-yr = 63 239.6717 AU With our new measuring sticks to hand we can give a few examples of the scale of the universe. The distance from the Earth to the nearest star (Alpha Centauri A or B) after our Sun is 4.3 ly. The Milky Way Galaxy is about 150,000 light-years across The andromeda galaxy is 2.3 million light-years away. The edge of the observable universe is 46.5 Giga light years away.THE PARSEC The other commonly used unit in astronomy and in Star Trek is called the Parsec (parallax of one arc second). The parsec is defined to be the distance at which a star would have a parallax angle p equal to one second of arc (1/3600 deg). The two dimensions that specify this triangle are the parallax angle (defined as 1 arcsecond) and the opposite side (defined as 1 Astronomical Unit (AU), the distance from the Earth to the Sun). The parsec defined as the distance required to create a parallax angle of one second of arc. Parallax is the apparent shift in the nearest stars due to the motion of the Earth around the Sun. The method of parallax gives rise to a natural distance unit that astronomers call the parsec (which we shall abbreviate as pc). The parsec in trigonometric terms. 1 Parsec = 3.08568025 × 1016 m. also used are kpc =1000 pc and Mpc =1 million pc 1 Parsec = 3.26 lt yrs. If the star is not further than 500 light-years, then the parallax shift of the star can be used to find the distance from the Earth. Distance (in parsecs) = 1/parallex angle.Magnitude of StarsApparent Magnitude Early Greek astronomers used a scale of magnitude devised by Hipparchus around the 2nd century BC, which was based on how bright stars appeared with the naked eye. The Hipparchus scale went from magnitude 1, for the brightest stars, up to magnitude 6, for those stars which were barely visible. Improvements in the light gathering power of telescopes made it possible to compare the intensities of the light from stars more accurately. In 1856, Norman Robert Pogson formalized the system by defining a typical first magnitude star as a star that is 100 times as bright as a typical sixth magnitude star. Thus, a first magnitude star is about 2.512 times as bright as a second magnitude star. The fifth root of 100 (since magnitude 6 stars must be 1: x5 is known as Pogson’s Ratio. In calculations, however, the factor 2.5 is often used. To make things more confusing, the brightest stars in the sky exceed magnitude 1. These bright starts are accommodated by allowing negative magnitudes. The Sun has an apparent magnitude of -26.74, while Sirius has a magnitude of -1.46. At the other end of the scale, as the light gathering power of telescopes has increased, the magnitude scale has extended to encompass much fainter stars. The dimmest object currently observable with the largest telescopes have a magnitude of 30. As a useful reference point, the star, Vega is taken to be of 0 magnitude. More accurate measurements put its apparent magnitude at 0.03. It is also important to note that the magnitude system is only meaningful when magnitudes are compared when measured through the same wavelength band.Name Apparent Magnitude Distance from Earth Sun -26.74 1 AU Full Moon - 12 200,000 km Venus -4.71 38 million km Sirius -1.46 2.6pc Vega 0.03 13pc Canopus 0.7 96pc ±5pc Faintest Stars 30 as seen with the European  Extremely Large Telescope (E-ELT) or Hubble Space  Telescope - The apparent magnitude m is given by m = - 2.5 log10(b) + C(1) Where, b is the observed intensity or brightness of the star and C is a constant, depending on the band the object is observed in, i.e. ultra-violet U, blue, B or visible V. If we measure the brightness of two different stars, using a detector in the same band, we can determine their difference in magnitude. The difference in their apparent magnitude is given by m1 - m2 = - 2.5 log10(b1/b2) where m1 and m2 two are apparent magnitudes of the two stars, and b1 and b2 are their respective brightness. From the properties of logarithms, the ratio of the intensities / brightness of the two stars is. m1 - m2 = - 2.5 [log10(b1) - log10(b2)] m1 - m2 = - 2.5 log10(b1/b2)(3)Absolute Magnitude The apparent brightness of a star is how bright it seems when viewed from the Earth, but a large, bright star can appear dim if it is a long way from the Earth and a dim star can appear to be bright if it is close to the Earth. Therefore, the apparent magnitude has no bearing on the distance from the Earth. To give an acurate measurement of the brightness of a star we need to make an absolute magnitude scale. The absolute magnitude is how bright a star is when viewed from a set distance. Stars being rather large objects, a distance of 10 parsecs was chosen. The absolute magnitude is the brightness of a star at a distance of 10 parsecs. Absolute Magnitude and Inverse Square Law of Intensity In order to find the absolute magnitude, we need to know the distance of the star from the Sun. How do we do this? The intensity or brightness of light decreases with distance from the star. The rate at which it decreases is inversely proportional to the square of the distance. Thus, if we have a star of luminosity L and we move a distance d the same quantity of light has to cover a larger spherical area. Therefore, the brightness or intensity is given by b = L/(4πd2) (4) Or in more simple terms, the apparent brightness of the star is proportional to the 1/distance2 To calculate the absolute magnitude we are essentially using the relative magnitude formula and the inverse square law to allow us to substitute distance for brightness. Now we can compare its magnitude with a star at set distance of 10pc. M- m = -2.5 log10(d2) - (-2.5 log10102) Using the rules of logarithms to make some simplifications. M = m -2.5 log10(d2/102) M = m - 5 log(d/10)(5) M = m - 5 [log(d) - 1] M = m - 5 log(d) + 5(6)Distance Modulus Starting from equation (6) we can calculate the distance d from the Earth if we know the absolute magnitude. In practice we don’t know the absolute magnitude because we cannot travel 10 parsecs from the star in question. We can use several indirect methods to determine its absolute magnitude. If the star is on the main sequence of stars then we can determine the brightness from its parallax. If we know the apparent magnitude m and the absolute magnitude then we can find the distance in parsecs to the star. m - M = 5 log10(d) + 5 Rearranging for d d = 10((m-M)+5)/5

atomstargazer:

Astronomical Distances and Magnitudes
Measuring Astronomical distances
In the words of Douglas Adams, the author of The Hitch-Hiker’s Guide to the Galaxy:

Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.
The distances involved in the universe are so vast that metres or kilometers will just not suffice. We must introduce new length scales with which can span the heavens.

THE ASTRONOMICAL UNIT A.U.

One natural and practical unit we can devise is the distance from the Sun to the Earth. This is the A.U. or Astronomical Unit. 1 Astronomical Unit = 149 598 000 km

LIGHT YEAR

Moving to larger distances even the AU becomes unweildly and so the next suitable unit is the light-year. The light-year, as its name would suggest, is the distance travelled by light in one year. All electromagnetic waves travel at a speed of x 299,792,458 ms-1 in a vacuum and with an average year being 365.25 days, one light year is 299,792,458 x 108ms-1 x (365.25 x 24 x 60 x 60) s =

9.46073 x 1015 m. or 9.46073 x 1012 km.

1 lt-yr = 63 239.6717 AU

With our new measuring sticks to hand we can give a few examples of the scale of the universe.

The distance from the Earth to the nearest star (Alpha Centauri A or B) after our Sun is 4.3 ly.

The Milky Way Galaxy is about 150,000 light-years across

The andromeda galaxy is 2.3 million light-years away.

The edge of the observable universe is 46.5 Giga light years away.

THE PARSEC

The other commonly used unit in astronomy and in Star Trek is called the Parsec (parallax of one arc second). The parsec is defined to be the distance at which a star would have a parallax angle p equal to one second of arc (1/3600 deg). The two dimensions that specify this triangle are the parallax angle (defined as 1 arcsecond) and the opposite side (defined as 1 Astronomical Unit (AU), the distance from the Earth to the Sun).

The parsec defined as the distance required to create a parallax angle of one second of arc.
Parallax is the apparent shift in the nearest stars due to the motion of the Earth around the Sun. The method of parallax gives rise to a natural distance unit that astronomers call the parsec (which we shall abbreviate as pc).

The parsec in trigonometric terms.
1 Parsec = 3.08568025 × 1016 m. also used are kpc =1000 pc and Mpc =1 million pc


1 Parsec = 3.26 lt yrs.

If the star is not further than 500 light-years, then the parallax shift of the star can be used to find the distance from the Earth.

Distance (in parsecs) = 1/parallex angle.

Magnitude of Stars
Apparent Magnitude
Early Greek astronomers used a scale of magnitude devised by Hipparchus around the 2nd century BC, which was based on how bright stars appeared with the naked eye. The Hipparchus scale went from magnitude 1, for the brightest stars, up to magnitude 6, for those stars which were barely visible.

Improvements in the light gathering power of telescopes made it possible to compare the intensities of the light from stars more accurately. In 1856, Norman Robert Pogson formalized the system by defining a typical first magnitude star as a star that is 100 times as bright as a typical sixth magnitude star. Thus, a first magnitude star is about 2.512 times as bright as a second magnitude star. The fifth root of 100 (since magnitude 6 stars must be 1: x5 is known as Pogson’s Ratio. In calculations, however, the factor 2.5 is often used.

To make things more confusing, the brightest stars in the sky exceed magnitude 1. These bright starts are accommodated by allowing negative magnitudes. The Sun has an apparent magnitude of -26.74, while Sirius has a magnitude of -1.46. At the other end of the scale, as the light gathering power of telescopes has increased, the magnitude scale has extended to encompass much fainter stars. The dimmest object currently observable with the largest telescopes have a magnitude of 30. As a useful reference point, the star, Vega is taken to be of 0 magnitude. More accurate measurements put its apparent magnitude at 0.03.

It is also important to note that the magnitude system is only meaningful when magnitudes are compared when measured through the same wavelength band.

Name Apparent Magnitude Distance from Earth
Sun -26.74 1 AU
Full Moon - 12 200,000 km
Venus -4.71 38 million km
Sirius -1.46 2.6pc
Vega 0.03 13pc
Canopus 0.7 96pc ±5pc
Faintest Stars 30 as seen with the European
Extremely Large Telescope (E-ELT) or Hubble Space
Telescope -
The apparent magnitude m is given by

m = - 2.5 log10(b) + C(1)

Where, b is the observed intensity or brightness of the star and C is a constant, depending on the band the object is observed in, i.e. ultra-violet U, blue, B or visible V.

If we measure the brightness of two different stars, using a detector in the same band, we can determine their difference in magnitude. The difference in their apparent magnitude is given by

m1 - m2 = - 2.5 log10(b1/b2)

where m1 and m2 two are apparent magnitudes of the two stars, and b1 and b2 are their respective brightness.

From the properties of logarithms, the ratio of the intensities / brightness of the two stars is.

m1 - m2 = - 2.5 [log10(b1) - log10(b2)]

m1 - m2 = - 2.5 log10(b1/b2)(3)

Absolute Magnitude
The apparent brightness of a star is how bright it seems when viewed from the Earth, but a large, bright star can appear dim if it is a long way from the Earth and a dim star can appear to be bright if it is close to the Earth. Therefore, the apparent magnitude has no bearing on the distance from the Earth.

To give an acurate measurement of the brightness of a star we need to make an absolute magnitude scale. The absolute magnitude is how bright a star is when viewed from a set distance. Stars being rather large objects, a distance of 10 parsecs was chosen.

The absolute magnitude is the brightness of a star at a distance of 10 parsecs.
Absolute Magnitude and Inverse Square Law of Intensity
In order to find the absolute magnitude, we need to know the distance of the star from the Sun. How do we do this? The intensity or brightness of light decreases with distance from the star. The rate at which it decreases is inversely proportional to the square of the distance. Thus, if we have a star of luminosity L and we move a distance d the same quantity of light has to cover a larger spherical area. Therefore, the brightness or intensity is given by
b = L/(4πd2) (4)

Or in more simple terms, the apparent brightness of the star is proportional to the 1/distance2

To calculate the absolute magnitude we are essentially using the relative magnitude formula and the inverse square law to allow us to substitute distance for brightness. Now we can compare its magnitude with a star at set distance of 10pc.

M- m = -2.5 log10(d2) - (-2.5 log10102)

Using the rules of logarithms to make some simplifications.

M = m -2.5 log10(d2/102)

M = m - 5 log(d/10)(5)

M = m - 5 [log(d) - 1]

M = m - 5 log(d) + 5(6)

Distance Modulus
Starting from equation (6) we can calculate the distance d from the Earth if we know the absolute magnitude. In practice we don’t know the absolute magnitude because we cannot travel 10 parsecs from the star in question. We can use several indirect methods to determine its absolute magnitude. If the star is on the main sequence of stars then we can determine the brightness from its parallax.

If we know the apparent magnitude m and the absolute magnitude then we can find the distance in parsecs to the star.

m - M = 5 log10(d) + 5

Rearranging for d

d = 10((m-M)+5)/5

, #Astronomy #physics #star #math #featured #astronomyfacts

electricspacekoolaid:

Puzzle of Spiral Galaxies Solved —“Self-perpetuating, Persistent, and Surprisingly Long Lived”

Some 15 percent of all galaxies in the visible Universe are spirals. The great fog-like clouds of stars, the oldest and largest galaxies in the Universe are ellipticals. Becasue ellipticals also include many of the smallest galaxies, they are the most numerous. Our own Milky Way, astronomers believe, is a spiral. Our solar system and Earth reside somewhere near one of its filamentous, swept-back arms. And nearly 70 percent of the galaxies closest to the Milky Way are spirals, suggesting they have taken the most ordinary of galactic forms in a universe with somewhere between 100 billion and 200 billion galaxies.

But a long-standing question has been: how do galaxies like the Milky Way get and maintain their characteristic arms has proved to be an enduring puzzle in astrophysics. How do the arms ofspiral galaxies arise? Do they change or come and go over time?*The answers to these and other questions are now coming into focus as researchers capitalize on powerful new computer simulations to follow the motions of as many as 100 million “stellar particles” as gravity and other astrophysical forces sculpt them into familiar galactic shapes.

Writing April 1 in The Astrophysical Journal, a team of researchers from the University of Wisconsin-Madison and Harvard-Smithsonian Center for Astrophysics report simulations that seem to resolve longstanding questions about the origin and life history of spiral arms in disk galaxies.

“We show for the first time that stellar spiral arms are not transient features, as claimed for several decades,” says UW-Madison astrophysicist Elena D’Onghia, who led the new research along with Harvard-Smithsonian Center for Astrophysics colleagues Mark Vogelsberger and Lars Hernquist. “They are self-perpetuating, persistent and surprisingly long lived.”

The origin and fate of the emblematic spiral arms in disk galaxies have been debated by astrophysicists for decades, with two theories predominating: One holds that the arms come and go over time. A second and widely held theory is that the material that makes up the arms – stars, gas and dust – is affected by differences in gravity and jams up, like cars at rush hour, sustaining the arms for long periods.

The new results fall somewhere in between the two theories and suggest that the arms arise in the first place as a result of the influence of giant molecular clouds, star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds, says D’Onghia, a UW-Madison professor of astronomy, act as “perturbers” and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.

“We find they are forming spiral arms,” explains D’Onghia. “Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there.”

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, #Science #space #galaxies #galaxy #Milky Way Galaxy #Milky Way #technology #astronomy #astrophysics #Universe #stars #star #astronomy facts #tech #cosmology #astronomyfacts
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