Astronomers have discovered a new type of star system: elongated, clumps of young, hot blue stars, which formed from gas ejected from the galaxy as it made its way through a galactic cluster.
Everything about this sentence is awesome
While these star systems were found on purpose – as astronomers were looking for – they turned out to be not what astronomers were expecting, which added a bit more fun to this whole story. [link to paper].
We live in the Milky Way, a huge galaxy that is part of a small group of a few dozen nearby galaxies that we call the Local Group. Radio wave surveys of the sky show many very small, fast-moving clouds of atomic hydrogen gas that are suspected to be either part of the Milky Way or the Local Group. Astronomers thought that these clouds might collapse to form stars, and in fact small galaxies.
However, initial visible-light observations didn’t come up much. They just looked like blue dots. But then deep observations of a single cloud showed that it was indeed forming stars. It was named SECCO-1, for the StEllar Counterpart of Compact High Speed Draw. However, they concluded that this was in fact not part of the Milky Way or the Local Group, but rather a very distant one, part of the Virgo Group of galaxies about 60 million light-years away!
The Virgo cluster contains thousands of large galaxies, and lots of gases floating around them. Encouraged by the initial results, more observations of these hydrogen clouds were made, using the Very Large Array for more detailed radio maps, the Very Large Large Telescope (or VLT) for spectroscopy, and the Hubble Space Telescope for images. They targeted gas clouds that also had ultraviolet emissions as seen by GALEX, indicating possible star formation.
They targeted five of these clouds. One of them turned out to be a distant galactic knot in the background, which is a bit disappointing. But the other four were transformed by Hubble into stars, showing that they are actually clumps of stars in space. While they can’t prove they’re part of a Virgo, their brightness and speed are compatible with that. It seems clear that they are already inside the block.
But what are they? They are strange. The bodies are lumpy and elongated and contain warm hydrogen gas consistent with star formation. All stars are blue, which indicates that they are small in size and massive. While it was originally found because it was seen in maps of atomic hydrogen, only one of them, called BC3, appears to contain much of this gas; The rest is deficient in it.
They don’t see bright red stars, which is another sign that systems are young. When stars die like the sun they expand into luminous red giants. This may take billions of years, but for massive stars it may only take a few tens of millions of years. Not seeing anything means these systems are really small.
It is also fairly low in mass, containing less than 100,000 times the mass of stars represented by the Sun. This may sound like a lot, but galaxies have millions or several billions of solar masses, and even globular clusters tend to have hundreds of thousands of solar masses. Globes are compact, a few light-years across, but these objects are several thousand to about 20,000 light-years long, and they are elongated.
So what are they? Clouds of gas floating in intergalactic space generally don’t collapse to form things like this. But this is not only intergalactic, but intergalactic In a huge galactic mass.
This environment makes a big difference. All galaxies in the cluster orbit each other due to their gravity. A gas-rich galaxy can move through the cluster at speeds of up to several hundred kilometers per second – millions of kilometers per hour – and when that happens it is subjected to a lot of pressure exerted by the thinner intergalactic gas. This can remove gas inside a fast-moving galaxy, in the same way you can ventilate a car while driving by opening the windows. This kind of thing is called ram pressure stripping.
We see a lot of this happening in many groups of galaxies including the Virgo Cluster, and in fact some galaxies are moving so fast that they lose a significant amount of gas. This orphan gas can compress when it explodes and move past its parent galaxy, forming stars as it does so. This may also explain why these new systems are elongated – a corollary to them by plowing through the gas within the cluster – and why they are also relatively isolated, not near any other galaxies. Their parent galaxies have long left them behind.
They have formed in a dramatic fashion, but their fate is fading away. Given its individual total mass, it is likely unrelated to gravity; In other words, over time, the stars in these systems will fly away, becoming isolated stars in the Virgo intergalactic cluster. Astronomers estimate that star clusters will last no more than 500 million years, a short period compared to the age of the galaxy cluster. It also means that these systems may be constantly forming as galaxies are being stripped of gas. If there was a single, short-term event making these things, they probably would have dissipated long ago.
If all of this is true – and the solid evidence points that way – then this really is a new type of star cluster. They form when gas erupts from galaxies that in turn form stars, eventually evaporating, over time permeating the group with millions of rogue stars.
This is amazing, and a great indication that we still have a lot to learn about the way the universe makes stars. Keep in mind that we only saw these things because they were close by; In distant gatherings they would be too weak to see clearly with current technology.
What else is happening in the corners of the universe that are too far away to be carefully examined?
It’s something of a fan
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