APOD: The Small Cloud of Magellan (2010 Sep 03)

Post a reply


This question is a means of preventing automated form submissions by spambots.
Smilies
:D :) :ssmile: :( :o :shock: :? 8-) :lol2: :x :P :oops: :cry: :evil: :roll: :wink: :!: :?: :idea: :arrow: :| :mrgreen:
View more smilies

BBCode is ON
[img] is ON
[url] is ON
Smilies are ON

Topic review
   

Expand view Topic review: APOD: The Small Cloud of Magellan (2010 Sep 03)

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by bystander » Fri Sep 03, 2010 8:58 pm

Discussion on BBCode split and merged with Help on using BBCode.

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by Ann » Fri Sep 03, 2010 6:02 pm

I can't resist talking a bit more about the nature of the stars in globular clusters.

Globular clusters are ancient, certainly those we find in the Local Group of galaxies. They are remnants of our galaxies' violent past, when huge gas clouds collided and gave rise to raging star formation and tremendously large and bright clusters. But that happened when the universe was very young, and no true globular clusters have formed in the Local Group for billions of years. Although there is a so called Populous Blue Cluster in the Large Magellanic Cloud:
NGC 1850, a young and extremely rich cluster in the Large Magellanic Cloud. (If you don't think that NGC 1850 looks particularly young and blue, that is partly because the color balance of the image is a bit red. Note, however, the red emission nebulosity around the cluster. This kind of thing is typical of young clusters, and you find nothing like that around any globular cluster in the Milky Way.)

So NGC 1850 is truly remarkably rich for a young cluster. But when today's globular clusters were young, they were much, much richer.

When a cluster is very young, all its stars may be on the main sequence. That means that all stars shine by converting hydrogen into helium in their cores. As long as the stars are on the main sequence the stars are bluer the brighter they are.
This is the young Pleiades cluster. Only the bright blue stars show up in this image. But the Pleiades contains at least three hundred stars, most of which are quite faint and yellowish. If we plot the brightness of the stars of the Pleiades versus the color of the stars of the Pleiades, we get a so called color-magnitude diagram, which in the case of the Pleiades looks like this:
The stars are brighter the higher up in the diagram they are. That is fairly easy to understand. But the stars are also bluer the farther to the left they are in the diagram. You can see that all the bright stars of the Pleiades are far to the left, so they are all blue. All the fainter stars are farther to the right. So the bright stars are blue and the faint stars are yellow, even reddish when it comes to the very faintest stars.

But this is a typical color-magnitude diagram for a young cluster, where all the stars are on the main sequence. As a cluster ages, its brightest, bluest stars exhaust the hydrogen in their cores, and then these stars will change color from blue to red. The most massive stars, which were originally the bluest, will also be the ones which burn out and die first.

Globular clusters are ancient. They don't have any truly massive stars left, no stars like the bright blue stars of the Pleiades. In an ancient globular cluster the brightest stars are the reddest. They are the ones which are about to burn out and die to become white dwarfs surrounded by planetary nebulae. Right before the stars cast off their atmospheres and become white dwarfs they grow monstrously large, with cool, tenuous and reddish atmospheres.

For a low metallicity globular cluster, the color-magnitude diagram looks something like this:
All the stars that are at the very bottom of the diagram are extremely faint, so faint that they don't contribute to the overall color of the cluster at all. The red stars at the bottom are really tiny, lightweight stars on the main sequence, little faint embers that will keep glowing for trillions of years because they burn their meager supplies of hydrogen so incredibly slowly. If these red little mini-stars had been any smaller, they wouldn't have been able to get their hydrogen fusion going at all.

As for the tiny stars to the left, the blue ones, they are hot white dwarfs, scorchingly hot naked cores of stars that have died and cast off all of their atmospheres. The "white" dwarfs are blue because they are so hot, and they are so faint because they are so extremely tiny.

You can see a broad but narrowing "road" of stars that seem to start at the lower right and rise towards the left, while the color of the stars change from red to orange to yellow. The brighter and more "non-red" the stars are, the more massive they are and the brighter they shine. The part of the "road" that "points to the left" is the main sequence. All the stars here shine by converting hydrogen to helium in their cores.

But after a while the broad "road" turns rightwards. The stars are getting brighter, but they are getting ever redder as they are doing so. The point at which the "road" turns to the right is, logically enough, the "turn-off point". This is the point at which the stars have used up the hydrogen in their cores. Remember that the most massive stars use up their hydrogen first, and then gradually the less and less massive stars follow. When the stars reach the turn-off point their central hydrogen is used up, and they have to produce energy by other means. At first their cores contract, which releases heat and makes the stellar atmospheres expand. As the stellar atmospheres expand the stars grow both bigger, brighter and redder. Now the stars have left the main sequence, never to return.

But the stars reach an intermediate stage where they contract again. They have now reached a stage where they produce more energy than they did when they were on the main sequence and burned only hydrogen, so they are brighter than when they were on the main sequence. But they are much smaller than they were when they were truly large. The fact that they have contracted so much, yet produce so much energy, makes them much bluer. You can find these blue stars fairly high up and to the left in the diagram. Note that the very bluest of these stars are fainter than the ones which are not so blue. That is because the bluest stars have become quite small, much smaller than the ones which are not so blue.

The stars in this "blue stage" are called "horizontal branch stars". But it is only in metal-poor clusters that the horizontal branch stars contract and heat up enough to be blue.

Below and to the right of the horizontal branch stars are the blue stragglers. They are the ones which have not "turned right" and left the main sequence when all the other stars of their mass have done so. Astronomers have thought of various reasons for why these stars "straggle" on the main sequence and are brighter and bluer than they "ought to" be. One possibility is that the blue stragglers are the products of stellar mergers in the crowded globular clusters.

But as you can see, the blue stragglers are neither as blue nor as bright as the blue horizontal branch stars.

After a while the stars leave the horizontal branch and begin their final ascent to the right. Now they become huge, bloated, and red. And then they shed their atmospheres, end their fusion processes, and die.

Ann

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by Ann » Fri Sep 03, 2010 4:53 pm

owlice wrote:Hubble, ESO, and other image-rich sources often offer images in different sizes. For a thread like this, where the images being posted are for illustrative purposes, I would like to suggest that images in the 300K and smaller range be used. I think this is particularly important in the Bridge, which sees the most traffic and is the forum most likely to have readers who may not have fast internet connections. Thanks.

To that end, I've replaced the links above with links to smaller images.
Thanks, Owlice.

Ann

The 2175-angstrom bump-PAH

by neufer » Fri Sep 03, 2010 4:45 pm

http://apod.nasa.gov/apod/ap100107.html wrote:
"The green color traces emission from organic dust grains
(mainly [P]olycyclic [A]romatic [H]ydrocarbons)
."

[Note: the PAH's are off to the side of the SMC BAR (in the "tail") and hence
don't contribute to a SMC absorption 2175-angstrom "bump-PAH"(; see below)]
http://asterisk.apod.com/vie ... 83#p114083
http://asterisk.apod.com/vie ... 85#p113785
http://asterisk.apod.com/vie ... 31&t=20929

http://antwrp.gsfc.nasa.gov/apod/ap080605.html

-------------------------------------
http://en.wikipedia.org/wiki/Extinction_%28astronomy%29 wrote:
<<Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation emitted by astronomical objects by matter (dust and gas) between the emitting object and the observer. The concept for interstellar extinction is generally attributed to Robert Julius Trumpler, though its effects were first identified in 1847 by Friedrich Georg Wilhelm von Struve.

Broadly speaking, interstellar extinction varies with wavelength such that the shorter the wavelength the stronger the extinction. Superimposed on this general trend are absorption features, which have various origins and can give clues as to the composition of dust grains. Known discrete absorptions features include (but not limited to) the 2175 Å bump, the diffuse interstellar bands, the 3.1 μm water ice feature, and the 10 and 18 μm silicate features.

The 2175-angstrom feature

One prominent feature in measured extinction curves of many objects within the Milky Way is a broad 'bump' at about 2175 Å, well into the ultraviolet region of the electromagnetic spectrum. This feature was first observed in the 1960s but its origin is still not well understood. Several models have been presented to account for this bump which include graphitic grains with a mixture of PAH molecules. Investigations of interstellar grains embedded in interplanetary dust particles (IDP) observed this feature and identified the carrier with organic carbon and amorphous silicates present in the grains.
The form of the standard extinction curve depends on the composition of the ISM, which varies from galaxy to galaxy. In the Local Group, the best-determined extinction curves are those of the Milky Way, the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). In the LMC, there is significant variation in the characertistics of the ultraviolet extinction with a weaker 2175 Å bump and stronger far-UV extinction in the region associated with the LMC2 supershell (near the 30 Doradus starbursting region) than seen elsewhere in the LMC and in the Milky Way. In the SMC BAR, more extreme variation is seen with NO 2175 Å and very strong far-UV extinction in the star forming Bar and fairly normal ultraviolet extinction seen in the more quiescent Wing. This gives clues as to the composition of the ISM in the various galaxies. Previously, the different average extinction curves in the Milky Way, LMC, and SMC were thought to be the result of the different metallicities of the three galaxies: the LMC's metallicity is about 40% of that of the Milky Way, while the SMC's is about 10%. Finding extinction curves in both the LMC and SMC which are similar to those found in the Milky Way and finding extinction curves in the Milky Way that look more like those found in the LMC2 supershell of the LMC and in the SMC Bar has given rise to a new interpretation. The variations in the curves seen in the Magellanic Clouds and Milky Way may instead be caused by processing of the dust grains by nearby star formation. This interpretation is supported by work in starburst galaxies (which are undergoing intense star formation episodes) that their dust lacks the 2175 Å bump.>>

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by owlice » Fri Sep 03, 2010 4:06 pm

Hubble, ESO, and other image-rich sources often offer images in different sizes. For a thread like this, where the images being posted are for illustrative purposes, I would like to suggest that images in the 300K and smaller range be used. I think this is particularly important in the Bridge, which sees the most traffic and is the forum most likely to have readers who may not have fast internet connections. Thanks.

To that end, I've replaced the links above with links to smaller images.

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by neufer » Fri Sep 03, 2010 3:18 pm

Should the Small Magellanic Cloud really be named Little America?
http://www.seds.org/messier/xtra/Bios/vespucci.html wrote:
<<During his third voyage in about 1503-4, Amerigo Vespucci mentioned in a letter describing the southern skies, "three Canopes [Navigational features?], two bright and one obscure." Amerigo's bright "Canopes" are thought to be the Magellanic Clouds, LMC and SMC, while the obscure one is probably the Coalsack dark nebula.>>
http://en.wikipedia.org/wiki/Amerigo_Vespucci wrote:
<<The [Third and] last certain voyage of Amerigo Vespucci (March 9, 1454 – February 22, 1512) was led by Gonçalo Coelho in 1501–1502 in the service of Portugal. On reaching the coast of Brazil, they sailed south along the coast of South America to Rio de Janeiro's bay. Portuguese maps of South America, created after the voyage of Coelho and Vespucci, do not show any land south of present-day Cananéia at 25° S, so this may represent the southernmost extent of their voyages.

After the first half of the expedition, Vespucci mapped Alpha and Beta Centauri, as well as the constellation Crux, the Southern Cross. Although these stars had been known to the ancient Greeks, gradual precession had lowered them below the European horizon so that they had been forgotten. On his return to Lisbon, Vespucci wrote in a letter to d'Medici that the land masses they explored were much larger than anticipated and different from the Asia described by Ptolemy or Marco Polo and therefore, must be a New World, that is, a previously unknown fourth continent, after Europe, Asia, and Africa.>>

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by Ann » Fri Sep 03, 2010 3:15 pm

As the Color Commentator, I ask you to note the difference in color between the Small Magellanic Cloud and 147 Tucanae. The SMC is made up mostly by young blue stars, but the color of the galaxy is further enhanced by the fact that the stars here are metal-poor. "Metal-poor" is astronomy-speak which means that the stars are made up of fairly (though not entirely) pristine hydrogen and helium. That is to say that the gas that made the stars in the SMC generally had not been "recycled" through many previous generations of stars. The stars in the Milky Way are generally much more metal-rich than the stars of the SMC, that is to say, "our" stars generally contain much higher levels of heavier elements like oxygen, carbon, silicon and iron. Our own Sun is quite metal-rich compared with most stars in the SMC. If the Sun had been as metal-poor as the stars of the SMC the Sun would have been bluer without being brighter. But it is also likely that the relative lack of heavy elements would have made it hard for rocky planets to form. In other words, if the Sun had been as metal-poor as the stars of the SMC, the Earth might not have existed!

So the SMC is quite metal-poor for a galaxy. But guess what, 47 Tuc is relatively metal-rich for a globular! The fact that the stars of 47 Tuc are relatively metal-rich also means that this globular cluster is relatively yellow as globulars go.

The reason why metal-poor globulars are bluer than more metal-rich ones is that an "intermediate" kind of star called horizontal stars are blue in the metal-poor clusters, but yellow in the metal-rich ones like 47 Tuc.

This is a typical metal-poor globular cluster, M13:
The colors here are somewhat enhanced. You should be able to spot the blue horizontal stars quite easily. They are not the brightest stars in the cluster, but they are bright enough, and they are numerous.

But this is 47 Tucanae. Take a look and compare it with M13:
There are no bright blue stars here, because the horizontal stars are all yellow. (There are some famous "blue stragglers" in 47 Tuc, but they are another type of stars and considerably fainter. Also they are not that numerous.)

Ann

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by emc » Fri Sep 03, 2010 12:27 pm

Hi Orin, Thanks! You’re a generous and kind man.

You know the universe is so amazing, especially from a human time frame perspective. The universe evolves over billions of years and yet we measure our time so much shorter. My sense of time makes it difficult for me to grasp cosmic time since my life window is so relatively tiny, at least it seems that way for me. I read that these star clouds orbit our galaxy over billions of years and I think… how incredible. Events that take billions of years, I mean. And there are so many incredible cosmic events before us and will likely be so many more after us… really makes me feel small… and quick!

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by orin stepanek » Fri Sep 03, 2010 12:03 pm

emc wrote:silence beckoned and I surrendered
clouds of stars outside the crowd
makes me wonder at their fate
shepherd… meander… onlookers… prey
what brings you to this domain today
are we like the star clouds
Hey Ed; nice poetry! 8-)

Re: APOD: The Small Cloud of Magellan (2010 Sep 03)

by emc » Fri Sep 03, 2010 11:51 am

silence beckoned and I surrendered
clouds of stars outside the crowd
makes me wonder at their fate
shepherd… meander… onlookers… prey
what brings you to this domain today
are we like the star clouds

APOD: The Small Cloud of Magellan (2010 Sep 03)

by APOD Robot » Fri Sep 03, 2010 4:05 am

Image The Small Cloud of Magellan

Explanation: Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two celestial wonders easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. These cosmic clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way Galaxy. The Small Magellanic Cloud actually spans 15,000 light-years or so and contains several hundred million stars. About 210,000 light-years away in the constellation Tucana, it is more distant than other known Milky Way satellite galaxies, including the Canis Major and Sagittarius Dwarf galaxies and the Large Magellanic Cloud. This sharp image also includes two foreground globular star clusters NGC 362 (bottom right) and 47 Tucanae. Spectacular 47 Tucanae is a mere 13,000 light-years away and seen here to the left of the Small Magellanic Cloud.

<< Previous APODDiscuss Any APOD Next APOD >>
[/b]

Top