Quantcast
Connect with us

A new family of stars in the Milky Way could help us work out how galaxies formed

Published

on

One of the most fundamental questions in modern astrophysics is how galaxies first formed. The Milky Way, the galaxy we live in, is an amalgamation of about 100 billion stars, gas, dust and enigmatic dark matter – all held together by gravity. The Milky Way itself is but one of many hundreds of billions of galaxies in the observable universe, with a wide range of sizes, masses, shapes and colours.

ADVERTISEMENT

We are just beginning to understand how such different kinds of galaxies could arise and evolve. But the picture is blurry. For example, cosmological models of galaxy formation haven’t been able to establish exactly where the stars we see today were first born. Figuring out how galaxies came to be is hugely important. Without them, there would be no stars – and therefore no life in the universe.

Now we have made a discovery, published online in the Monthly Notices of the Royal Astronomical Society, that sheds new light on how galaxies and stars within them form.

Enigmatic globular clusters

Astronomers know that some of the Milky Way’s stars were born in the galaxy itself, whereas others were formed in smaller galaxies that the Milky Way eventually engulfed. But it is unclear which process is most common.

Infant stars are born in large groups – never in isolation. Some of these groups, called clusters, are still around today. They roughly come in two types: young, low-mass clusters that live in the Milky Way disk, and massive, old “globular clusters” which are located in the “stellar halo” of the Milky Way (a very large, thin, diaphanous structure) which envelops the disk of the Milky Way. Globular clusters contain a tiny fraction of all Milky Way stars, but astronomers suspect that they hold important clues to the early stages of how the galaxy formed.

ADVERTISEMENT

Image of the disk and halo of the Milky Way by COBE in infrared. NASA

 

But we don’t really know how globular clusters formed either. Current models propose that most globular clusters formed in giant molecular clouds within turbulent disks in the early universe. They suggest that globular clusters host multiple generations of stars – stars born from material of other stars, some of which are now dead. However, these models cannot reproduce in detail what we actually observe in globular clusters.

What we know is that the universe only assembled the conditions to form lots of globular clusters in its youth. Ironically, these turbulent disks were also hostile to globular clusters – it is thought that most of them were wiped out by collisions with giant molecular clouds shortly after being born. Those that survived (there are about 150 in the Milky Way) may have lost a substantial fraction of their stars, perhaps even most of them. But if that’s the case, then there should be plenty of stars originally formed in globular clusters now residing in other parts of the galaxy.

ADVERTISEMENT

The answer could be written in the stars themselves. Their locations, velocities and chemical compositions may hold clues to whether these stars were formed in the Milky Way or elsewhere. All the chemical elements in the universe, except for hydrogen, helium, and tiny amounts of lithium, were synthesised entirely in the interior of stars. When stars die, they shed their nucleosynthetic byproducts into the interstellar medium, out of which new stars are born. Thus, a stars’s chemical composition bears the signature of the evolution of its predecessors. So if we can work that out, it may be possible to establish a star’s origin.

The galaxy has many many stars, though, so to obtain meaningful answers, one needs very large samples. Thanks to recent technological developments, huge surveys currently collect data for hundreds of thousands to millions of Milky Way stars. One of them is the Apache Point Observatory Galactic Evolution Experiment (APOGEE). It stands out from the crowd because of its focus on infrared spectroscopy – a lower frequency of light than that we can see – as opposed to all other surveys that concentrate in the visible part of the spectrum.

This is important because of all the dust in the disk of the Milky Way. Dust is a lot less opaque in the infrared, so APOGEE can see through the dust into the inner regions of the Milky Way much better than optical surveys. This enabled us to estimate the abundances of some key elements for thousands of stars for the first time. We were also able to detect trace stellar families that would otherwise pass unnoticed amid a crowd of ordinary stars.

ADVERTISEMENT

Peculiar stars

In this way, we discovered a population of stars with unusual chemical compositions, very enriched in nitrogen. This is interesting because it is typical of globular cluster stars. The properties of these new stars differ from those of existing globular clusters, suggesting that they were associated with clusters that no longer exist. The logical implication is that there once existed a very large population of globular clusters in the early Milky Way, which was almost entirely destroyed. Moreover, the properties of the new stars suggest that they are associated with the halo of the Milky Way, not its disk. That being the case, destroyed globular clusters may be the stuff that at least a quarter of the halo is made of.

The Messier 80 globular cluster in the constellation Scorpius. NASA

 

If confirmed, this result will challenge models of galaxy formation. For instance, APOGEE’s result can constrain which fraction of the halo was formed in the galaxy. It also forces us to revise models of globular cluster formation, which have a hard time explaining the amount of nitrogen that we observe in these stars.

ADVERTISEMENT

But perhaps a more far-reaching implication is that we might discover that globular clusters are in fact one of the the most typical star-forming units in the universe. In the past two decades, research has made it possible to estimate the mean chemical composition of stars in the cores of distant, unresolved elliptical galaxies, which are thought to be formed in similar ways to the Milky Way halo. Interestingly, this has shown that stars in those galaxies tend to be enriched in nitrogen and sodium, which is precisely what is found in globular clusters.

Indeed, globular clusters may have contributed substantially to the stellar budget of all galaxies in the universe – something we didn’t know before. This is a riveting prospect which could even come to change our understanding of how galaxies came about, including our own Milky Way.

The Conversation

By Ricardo Schiavon, Reader in Astrophysics, Liverpool John Moores University

ADVERTISEMENT

This article was originally published on The Conversation. Read the original article.


Report typos and corrections to: [email protected].
READ COMMENTS - JOIN THE DISCUSSION
Continue Reading

Breaking Banner

Ukrainian journalist throws down gauntlet after Giuliani smear: ‘I express my readiness to testify’

Published

on

A Ukrainian journalist said on Sunday that he would be willing to testify to Congress against President Donald Trump's attorney, Rudy Giuliani.

After Giuliani unleashed a bizarre rant on CNN accusing Democrats of trying to get help from Ukraine in the 2016 election, Ukrainian journalist Serhiy Leshchenko wrote an op-ed exposing the accusation as a lie.

In his op-ed, Leshchenko explains:

Never in my wildest dreams did I imagine the Manafort revelations would become fodder for the U.S. elections in 2020. President Trump’s lawyer Rudolph W. Giuliani, the mouthpiece of this campaign, is not only attempting to rehabilitate Manafort but is also working to undermine U.S. relations with Ukraine, which has been confronting Russian aggression on its own for more than five years. Giuliani and his associates are trying to drag our newly elected president, Volodymyr Zelensky, into a conflict between two foreign political parties, drastically limiting Ukraine’s room for maneuver in respect to the United States, perhaps its most important international partner.

Continue Reading

Breaking Banner

Trump is openly colluding with Ukraine to smear Biden

Published

on

President Donald Trump is defending his phone call with Ukrainian President Volodymyr Zelensky, one that recent media reports suggest may have been made in order to dig up dirt about one of Trump's likeliest and strongest opponents in the 2020 election, former Vice President Joe Biden.

This article first appeared on Salon.

"The Fake News Media and their partner, the Democrat Party, want to stay as far away as possible from the Joe Biden demand that the Ukrainian Government fire a prosecutor who was investigating his son, or they won’t get a very large amount of U.S. money, so they fabricate a story about me and a perfectly fine and routine conversation I had with the new President of the Ukraine," Trump tweeted on Saturday. "Nothing was said that was in any way wrong, but Biden’s demand, on the other hand, was a complete and total disaster. The Fake News knows this but doesn’t want to report!"

Continue Reading
 

Breaking Banner

Democrats’ refusal to impeach Trump is a ‘bigger national scandal’ than him breaking the law: Ocasio-Cortez

Published

on

Rep. Alexandria Ocasio-Cortez, D-N.Y., criticized House Democrats on Saturday for their unwillingness to impeach President Donald Trump despite new revelations that he may have pressured Ukraine to dig up dirt on one of his potential Democratic rivals in 2020, former Vice President Joe Biden.

This article first appeared in Salon.

"At this point, the bigger national scandal isn’t the president’s lawbreaking behavior - it is the Democratic Party’s refusal to impeach him for it," Ocasio-Cortez tweeted on Saturday night.

Continue Reading
 
 
Help Raw Story Investigate and Uncover Injustice. Join Raw Story Investigates for $1 and go ad-free.
close-image