Second-Generation Star Supports Cannibal Theory of Milky Way

Article written: 3 Mar , 2010
Updated: 24 Dec , 2015
by

[/caption]

A newly discovered red giant star is a relic from the early universe — a star that may have been among the second generation of stars to form after the Big Bang. Located in the dwarf galaxy Sculptor some 290,000 light-years away, the star has a remarkably similar chemical make-up to the Milky Way’s oldest stars. Its presence supports the theory that our galaxy underwent a “cannibal” phase, growing to its current size by swallowing dwarf galaxies and other galactic building blocks.

“This star likely is almost as old as the universe itself,” said astronomer Anna Frebel of the Harvard-Smithsonian Center for Astrophysics, lead author of the Nature paper reporting the finding.

Dwarf galaxies are small galaxies with just a few billion stars, compared to hundreds of billions in the Milky Way. In the “bottom-up model” of galaxy formation, large galaxies attained their size over
billions of years by absorbing their smaller neighbors.

“If you watched a time-lapse movie of our galaxy, you would see a swarm of dwarf galaxies buzzing around it like bees around a beehive,” explained Frebel. “Over time, those galaxies smashed together and mingled their stars to make one large galaxy — the Milky Way.”

If dwarf galaxies are indeed the building blocks of larger galaxies, then the same kinds of stars should be found in both kinds of galaxies, especially in the case of old, “metal-poor” stars. To astronomers, “metals” are chemical elements heavier than hydrogen or helium. Because they are products of stellar evolution, metals were rare in the early Universe, and so old stars tend to be metal-poor.

Old stars in the Milky Way’s halo can be extremely metal-poor, with metal abundances 100,000 times poorer than in the Sun, which is a typical younger, metal-rich star. Surveys over the past decade have
failed to turn up any such extremely metal-poor stars in dwarf galaxies, however.

“The Milky Way seemed to have stars that were much more primitive than any of the stars in any of the dwarf galaxies,” says co-author Josh Simon of the Observatories of the Carnegie Institution. “If dwarf
galaxies were the original components of the Milky Way, then it’s hard to understand why they wouldn’t have similar stars.”

The team suspected that the methods used to find metal-poor stars in dwarf galaxies were biased in a way that caused the surveys to miss the most metal-poor stars. Team member Evan Kirby, a Caltech
astronomer, developed a method to estimate the metal abundances of large numbers of stars at a time, making it possible to efficiently search for the most metal-poor stars in dwarf galaxies.

“This was harder than finding a needle in a haystack. We needed to find a needle in a stack of needles,” said Kirby. “We sorted through hundreds of candidates to find our target.”

Among stars he found in the Sculptor dwarf galaxy was one faint, 18th-magnitude speck designated S1020549. Spectroscopic measurements of the star’s light with Carnegie’s Magellan-Clay telescope in Las Campanas, Chile, determined it to have a metal abundance 6,000 times lower than that of the Sun; this is five times lower than any other star found so far in a dwarf galaxy.

The researchers measured S1020549’s total metal abundance from elements such as magnesium, calcium, titanium, and iron. The overall abundance pattern resembles those of old Milky Way stars, lending the first observational support to the idea that these galactic stars originally formed in dwarf galaxies.

The researchers expect that further searches will discover additional metal-poor stars in dwarf galaxies, although the distance and faintness of the stars pose a challenge for current optical telescopes. The next generation of extremely large optical telescopes, such as the proposed 24.5-meter Giant Magellan Telescope, equipped with high-resolution spectrographs, will open up a new window for studying the growth of galaxies through the chemistries of their stars.

In the meantime, says Simon, the extremely low metal abundance in S1020549 study marks a significant step towards understanding how our galaxy was assembled. “The original idea that the halo of the Milky
Way was formed by destroying a lot of dwarf galaxies does indeed appear to be correct.”

Source: Harvard-Smithsonian Center for Astrophysics

, ,



20 Responses

  1. matchu322 says

    hmm, maybe in the coming months, this would make a great podcast. Good work everyone

  2. erthx13 says

    Could the ratio of metal-poor against metal-rich, younger stars tell us something about the age of dwarf galaxies ?

    Am i very lost ?

  3. Jon Hanford says

    That’s a very insightful question, erthx13. Using spectra from stars in a galaxy helps astronomers deduce the type of star they are observing, hence their age. In some dwarf galaxies there are populations of stars with different metallicities or other characterizing attributes, sometimes showing punctuated eras of star formation. Metallicity differences occur in galaxies, galaxy clusters, globular and a few open clusters, supernovae, nova, etc.

    In short, hi-low metallicity maps are used to look at numerous objects and help discern their nature.

  4. erthx13 says

    Thank you, i appreciate it.

    I found this very fresh paper, i think it is related:
    The Galactic structure and chemical evolution traced by the population of planetary nebulae
    http://arxiv.org/abs/1003.0759

  5. Lawrence B. Crowell says

    I have a question somewhat related to erthx13’s. Large red giant stars are to my mind rather short lived stars. It has been my understanding these have much shorter life times than F through M class, or white to red dwarf, stars. It then strikes me as a bit odd this star could represent old PopII stars from remnants of dwarf galaxies which formed the Milky Way galaxy many billions of years ago.

    LC

  6. DrFlimmer says

    @ LBC:

    I think you confuse large red stars with large blue stars. The blue ones are those that go BANG shortly after their first ignition.

    The red ones are the elder versions of stars like our sun, undergoing helium burning or other forms. They will finally turn into White Dwarfs.

  7. Lawrence B. Crowell says

    So I presume this star is a fairly ordinary star just at the end of its life.

    LC

  8. Hon. Salacious B. Crumb says

    Some oddities here with the replies, but this explanation should help…

    Firstly, stars don’t form into large red giant stars, they evolve that way. Stars mostly go through their lives on the main sequence, being stars ranging from wee low-massed red dwarf, smaller cooler K and G-type stars, up through to the F and A spectral types. Here they spend most of their lives (c.80%) burning their hydrogen at their cores at a regular rate.
    After this phase, the fuel begins to run out, and the core shrinks by gravitation, but this increases the heat, so these star begin to swell and the atmosphere expands, to eventually forming red giants. This means the stars have moved away from the main sequence. Here the luminosity eventually increase in a straight line called the Asymptotic Giant Branch (AGB), end at a similar place for most main sequence stars.
    In the star they have found, was probably once a star one or two times the mass of the sun, which has gone through its evolution and now appears as a red giant.
    Now one thing strange about stars is that their initial composition are purer sources of hydrogen (X parameter) and helium (Y parameter) than say stars like the sun. The other elements (heavier than helium) of the “metals”, sometimes called the ‘Z’ parameter”, have significant effects on how the core burns its hydrogen and helium. The higher the metal content the bigger the effect. One of the biggest influences is on the lifetime of the star, which increases with metal content. The purer the hydrogen and helium in the star, the more efficiently the fusion process work – meaning they are theoretically more luminous and can be larger masses than star of similar ages to the sun.
    That the basic stuff.
    Now one of the other properties is that purer stars tend not to make as many metals than more ‘metallic’ one. When you look at the surface spectra of the star, they show significantly less metals than an equivalent aged and massed star. This signature tell how young or old stars are in cosmic terms. (It tells for instance the difference between metal poor Pop I stars (like in globulars, centre of galaxies, and elliptical galaxies) and metal rich Pop II stars (formed in spiral arms and galaxy interaction or collision.)
    Finally, so called population III stars were the first stars born in the universe, whose discovery is yet to be made. A Pop III’s equivalent metal signature has the spectra with virtually no sign of other metals.
    In the end, chemical ‘metal’ enrichment increases over time, which in turn tells us something of the region in which the star formed and the ‘purity’ of the galaxy observed.

    That should answer the question here….

    (Suggest you read some more on stellar evolution, it might help with the more complex stuff like star formation and the deaths of stars I.e. As black holes, neutron stars, white dwarfs, etc..)

    Note: Nancy, we clearly need more stellar evolution articles and perhaps a slightly deep explanation of basic evolution theory.

  9. William928 says

    @HSBC:

    I don’t think we need Nancy to provide more articles, you did a fine job with your post on stellar evolution. Thanks!

  10. erthx13 says

    Indeed, very nice answer 🙂

    I did not know about this, thank you :
    ‘One of the biggest influences is on the lifetime of the star, which increases with metal content.’

    Just one more question: Where do dwarf galaxies come from ? How do they form, from what and when ? I know about galaxy core ejections, but i’am not sure if that is enough ?

    If the answer is too long, i can read Wiki, or other articles too, just point me 😉

  11. Jon Hanford says

    The extremely low metallicity of these stars may also play several parts in the final stellar/brown object.

  12. Jon Hanford says

    @Dr Flimmer sez: “The red ones are the elder versions of stars like our sun, undergoing helium burning or other forms. They will finally turn into White Dwarfs.”

    I know you were referring to red giants expelling their outer layers to lay bare the WD nucleus. But, after that, the dwarf cools and darkens as a glowing ember over many Gyr. Just a (slight) clarification. 🙂

    BTW: some great info on these UT blogs these days (about time) and thanks also to those whose expertise make this site work. (You know who you are).

  13. Jon Hanford says

    Forgot to mention, red dwarfs are the major stellar component to this, or other, galaxies. And they certainly are not all supernovae remnants cooled down. Some formed as low-mass red dwarf stars.

  14. Lawrence B. Crowell says

    I guess I have still further questions. I was under the impression that some red giants were stars some 50 to up to 100 times the mass of the sun, such as I believe betelguese. Again bear with me, for I only know stellar astrophys 101 stuff. So I have thought there were two types of red giant stars. The first kind are large massive stars which have settled into a red giant configuration off the main sequence, and main sequence stars which bloat up at the end of their life.

    LC

  15. Hon. Salacious B. Crumb says

    erthx13 said;

    “Just one more question: Where do dwarf galaxies come from ? How do they form, from what and when ? I know about galaxy core ejections, but i’am not sure if that is enough ?”

    Actually, we don’t know where dwarf galaxies come from, but we have the suspicion that they formed in vast numbers in the early universe. Some think, as stated in this article, they were the precursors of all the galaxies we see today – accumulations of many dwarf galaxies.
    One could assume that the dwarf galaxies once formed in vast nebulae – just like the star clusters – but only on a bigger scale. This is unlike the view that todays galaxies formed at once, which is rejected as unlikely based on the difficulties of nebulae across many kiloparsecs forming over such a very short period. Yet we know that similar galaxies we see today exist in the very earliest times in the universe, so some mechanism has to be quick enough to create them.
    If we could find real evidence of the first generation of stars or metal-poor stars, most astronomers believe we could understand this process better. Why? Then we would understand how the galaxies were so quickly enriched with these ‘metals’, and able to make things like solar-like stars and planets, and also their general structure and their varied morphologies..
    As to “galaxy core ejections”. I am not sure that is actually relevant to your question here.

  16. erthx13 says

    Thank you again 🙂

  17. Hon. Salacious B. Crumb says

    Lawrence B. Crowell said;

    “I was under the impression that some red giants were stars some 50 to up to 100 times the mass of the sun, such as I believe Betelguese. Again bear with me, for I only know stellar astrophys 101 stuff. So I have thought there were two types of red giant stars. The first kind are large massive stars which have settled into a red giant configuration off the main sequence, and main sequence stars which bloat up at the end of their life.

    Actually this is a common misconception. Red giants are named that way not because of their mass but there physical size.
    No known red giants / red supergiants exist above above 20 solar masses (Betelgeuse is about 15 to 18 Solar Masses, and is considered a progenitor of the the red supergiants.), because they blow up as supernovae before they reach that stage. In the H-R Diagram, when they reach the main sequences they immediately move across the diagram (right) in very quick time.

    NOTE: This is just as exactly Dr.Flimmer rightly says; “I think you confuse large red stars with large blue stars. The blue ones are those that go BANG shortly after their first ignition..
    It is exactly like the wikipedia entry on “Stellar Evolution” properly says;

    Extremely massive stars (approximately >40 solar masses), which are very luminous and thus have very rapid stellar winds, lose mass so rapidly due to radiation pressure that they tend to strip off their own envelopes before they can expand to become red supergiants, and thus retain extremely high surface temperatures (and blue-white color) from their main sequence time onwards.

    Also red stars, except for the little red dwarfs which don’t have sufficient mass to shine as brightly (and are not main sequence stars and are technically known as sub-dwarf stars), are not formed but have evolved to that state. (I.e. All average main sequence stars (0.8 to 12-15 solar masses) will end their lives as red giants or supergiant stars. They typically scale up along the Asymptotic Giant Branch (AGB), then they lose they mass through stellar winds, exposing the core of the star (up to 80%), then become planetary nebula over some 50,000 years, and end as a hot white dwarf stars.

    I’d recommend you read something like James B. Kaler’s “Stars and Their Spectra : An Introduction to the Spectral Sequence” (1989) See Cambridge University Press Preview. [Page 29-30 gives an excellent broad-brush summary of star evolution.

    Note: the quote of “50 to up to 100 times the mass of the sun”, is actual quoted by Kaler for size (diameter) NOT by mass. (1st Paragraph on pg.30)

  18. erthx13 says

    Here are two articles on the Wiki, which i should ve probably read before asking ..
    http://en.wikipedia.org/wiki/Dwarf_galaxy
    http://en.wikipedia.org/wiki/Galaxy_formation_and_evolution

    Thank you for your patience 🙂

  19. Jon Hanford says

    FWIW, I attended several packed lectures given by Jim Kaler at OSU in the late 70s-early 80s. I wish I has a cell phone camera for those lectures! Kaler is one of the greats when it comes to stellar classification and was pleased to find he has a friendly, easygoing manner and it was my privilege to meet him in person and get a signed copy of “Stars and Their Spectra…” (still have my copy), used by many a budding astronomer as an intro to the world of stellar astronomy. It is still one of my favorite books on the matter. I was impressed by his nature and his encyclopedic knowledge. A living legend.

  20. Hon. Salacious B. Crumb says

    The more detailed article and data, with reference to the original paper, can be viewed as; Pictures for press coverage of paper “Linking dwarf galaxies to
    halo building blocks with the most metal-poor star in Sculptor”

    The other concerning thing is this story is almost three months old. The issue seems not to be the news commentary but from the Harvard team who produced the paper. Why did they do this/ Were they ‘testing the waters’ and see the reaction before publicising it? All very strange. (a story in itself!)

Comments are closed.