Spiral Galaxy UGC 9684 – Star Factory
It seems there was such a popular music show. But now we are talking about gigantic gas balls, extremely hot, in which a significant part of the universal matter is concentrated, and inside which this matter evolves, acquiring a very wide variety, covering almost half of the periodic table of Mendeleev. We are talking about such stars, and not about any others, although there is an extra-scientific idea that the soul, going through the school of growing up in the human body, was once only an atom, but having gone through most of its spiritual evolution – through millions of incarnations – will become a star…
But let's return to astronomy.
IN previous story I have already talked about the galaxy M100, which is relatively close to us, which was photographed by an amateur astronomer Drew Evansgetting with his beautiful photo on the front page of the APOD website – Astronomy Picture of the Day May 2 this year. The M100 galaxy was characterized by a fairly high rate of star formation and numerous supernova explosions.
Today the world community is already discussing another, but somewhat similar galaxy – UGC 9684, and in some places in the headlines articles The following metaphor is mentioned about her: “A Star Forming Factory”. Attention is drawn to the photograph, which is by no means an amateur one. This spiral galaxy was photographed by the Hubble Space Telescope; it is not accessible to amateurs. Or, at least, amateurs would not be able to capture it in such detail.
First of all, what is this strange designation – UGC 9684?
People who are partial to the sky have a good idea of the catalog of Charles Messier, the 18th-century French comet catcher. Charles did a great job – he was the first to collect in one table all the objects of an incomprehensible nature, most of which had a diffuse appearance. It included nebulae, star clusters of various types, galaxies, and the numbers of all objects in this catalog were accompanied by the Latin letter “M”. But astronomers did not realize the gulf of differences between these cosmic formations.
The work of Charles Messier was continued by William Herschel, and later by his son John Herschel – the coverage of these new catalogs exceeded Messier's work tens of times, but their structure turned out to be inconvenient, and now we only sometimes remember them, but never use them. John Dreyer corrected the matter by reworking the Herschel legacy and adding thousands of newly discovered objects to it. This is how the New General Catalog appeared – NGC, which has become a classic – it is still used by both professionals and amateurs, and has a number of additions (IC – Index Catalog, focused on objects of the southern celestial hemisphere, historically less studied). But among the 14 thousand objects, there was still no morphological difference – nebulae, clusters of stars, and galaxies were present here on an equal footing. However, in the 19th century, any astronomer would have asked you in surprise: “What? What kind of galaxies?!”
Obviously, new times required the creation of new catalogs.
UGC is one of such fundamental catalogs, which contains only galaxies, but only of the northern hemisphere.
UGC stands for Uppsala General Catalog (of galaxies). It was created by employees of the Uppsala Observatory (Sweden), but largely relies on a photographic review of the Palomar Observatory (USA) and the “Catalogue of Galaxies and Their Clusters” by the Swiss astronomer Fritz Zwicky. The UGC contains nearly 15 thousand galaxies in the northern sky, the integral brightness of which exceeds 14.5 stellar magnitude.
The galaxy that we are talking about today is just close to the brightness limit for the UGC catalogue. Its integral magnitude is 14.4m – at the very edge of visibility of the Palomar Survey maps. Such a galaxy cannot be photographed beautifully using amateur means.
But the Hubble Telescope can.
What did he see there? Let's take a look too.
At first glance, it is an ordinary spiral galaxy located in the constellation Boötes (near the direction of the north pole of the Milky Way, which means that from there our Galaxy would be visible almost flat). It is very distant – 250 million light years (five times further than M100 – about which previous story), and 100 times further than the Andromeda Galaxy, which is closest to us. And in size, the galaxy UGC 9684 is almost the same as M100 and the Milky Way – these are all galaxies of approximately equal weight categories. The Andromeda Galaxy (M31) can be classified in the same category.
But there are also some distinctive features. UGC 9684 is a galaxy with a “bar”, or with a bar. The Milky Way has one. M100 and M31 do not have a jumper. And if you look at the spiral arms of UGC 9684, they reveal a rather confused structure in the middle part of the galactic disk – not clearly defined. But there are also external spiral branches – dim, but sweeping. It looks as if a bright and structurally dense spiral galaxy is immersed in a more voluminous, but rarefied, and also spiral galaxy – the Matryoshka Galaxy.
There is an assumption that the outer spiral branches began to form much later than the inner ones, and are a very young part of the galaxy UGC 9684. The fact is that each spiral galaxy has a clearly visible part: a core, a bulge (a spheroidal compaction enveloping the galactic core), spiral branches , the jumper between the branches and the core (if there is one at all). But there is also an invisible part of the galaxy – a huge spheroidal halo – like a giant bubble that surrounds the entire galaxy and is several times larger in size. It’s so easy – in a photograph – that the galactic halo cannot be seen. But ultrasensitive photometers detect its presence in almost all spiral galaxies.
What's in the halo?
First of all, hydrogen. Lots of hydrogen. It may turn out that there is as much hydrogen in the halo as in the galactic disk – in the spiral arms, core and bulge – in the form of nebulae and in the composition of stars. In fact, a galactic halo is like another galaxy, only unmanifested. Rare stars also wander in the halo. The density of the stellar population in the galactic halo is quite low, but the halo is large, and in the end it turns out that a significant part of the stars of each spiral galaxy (from 10 to 30%) can be located in the halo. And here also lie the orbits of globular star clusters, of which there are not many – several hundred or thousand in each galaxy. Many of these clusters are remnants of the cores of the galaxies that merged to form the one we now observe. You can simply count the globular clusters in a certain galaxy and find out the approximate number of galactic mergers in its history. But of course, the fullness of the halo of each spiral galaxy is very different.
Apparently, the halo of the galaxy UGC 9684 has significant star formation potential, and right before our eyes, a second tier of spiral branches is being born from it. This is something like how a large metropolis is growing with new microdistricts around its ring road (this is an allusion to the well-known city of Russia).
What was the impetus for the awakening of such a rapid growth of the stellar population – this is what astronomers working with the Hubble Telescope are trying to find out. Plus, they were puzzled by the fact that in the last 18 years alone, 4 supernovas have erupted in this galaxy – in 2006, 2012, 2017, 2020. And of course, now they look at this galaxy from the Milky Way very often, so as not to miss more one supernova, because such a machine-gun burst of supernovae cannot be an accident. It’s only a pity that we didn’t look here before – there were no technical means, and how can one guess in which of the tens of thousands of galaxies available for observation something interesting is happening right now… (although, the expression “right now” It’s hardly appropriate here.)
I mentioned that the halo of spiral galaxies is almost transparent and hardly reveals itself at all – through it we see everything that happens behind it. But the spiral branches, too, like a light veil, almost do not delay the light of more distant galaxies. Of course, they contain enough dust, but not so much as to be a barrier to the radiation of even more distant galaxies. See how even more distant star cities are clearly visible through the spiral arms of the galaxy UGC 9684.
They differ significantly in color – distinctly orange. This, friends, is the same red shift that indicates the total expansion of the Universe. Objects moving away from us send us their rays in a stretched form, like an accordion or a spring. The frequency of this radiation is reduced (the wavelength is correspondingly increased), which means the radiation itself is shifted to the red side of the spectrum. At such distances, a spectrograph is not needed – everything is obvious. 100 years ago, Edwin Hubble and his contemporaries measured subtle shifts in the main absorption lines in galactic spectra and drew dramatic conclusions from the barely observed effect. But now observational astronomy has reached a point where the red shift is visible simply on a photo card with the eye – is the galaxy red? – it means it is moving away quickly, it means it is very far from us.
There is another way to estimate how far those orange-red star cities are. They are approximately how many times further away, how many times smaller in apparent size than the galaxy UGC 9684. Of course, all galaxies are different, and by reasoning this way we risk being mistaken by a factor of two or three, but not by an order of magnitude. In addition, the size of this galaxy, like the size of the Milky Way – 100 thousand light years – is quite typical for spiral star islands. And if some galaxy is significantly smaller, then it is less visible – most likely, with such a redshift value, we would not have seen it in this photo.
According to redshift measurements, the galaxy UGC 9684 is moving away from us at a speed of about 5 thousand kilometers per second. The speed of removal of galaxies visible behind it is significantly higher – tens of times.
According to scientists' estimates, in the galaxy UGC 9684 – as we now see it – an average of one solar-mass star is born every few years. This is a fairly high rate of star formation for a spiral galaxy of this size. But for one supernova (with a mass of several suns) to burst out every few years, the rate of star formation in the past would have to be significantly higher. What happened there several hundred million years ago? – this is still a mystery for astronomers. One of the most understood mechanisms that activate the process of the birth of new stars is the merger of galaxies. But perhaps in the life of each galaxy there are some additional rhythms and cycles that influence the birth and death of stars. This remains to be seen.
