Object 105: Sagittarius Dwarf Irregular Galaxy

Podcast release date: 21 August 2023

Right ascension: 19:29:59.0


Epoch: ICRS

Constellation: Sagittarius

Corresponding Earth location: An area near the Rio Desaguadero in Bolivia about 10 km south of the town of Eucaliptus

This episode's coordinates point to the Sagittarius Dwarf Irregular Galaxy, which, as you may guess, is located in the constellation Orion. Just kidding, it's located in the constellation Sagittarius. This galaxy was discovered the old fashioned way: by people staring really really hard at photographic plates. The first paper about the galaxy was published in 1977 by a group led by Diego A. Cesarsky [1] and then rediscovered by a different group led by Andrew Longmore the next year [2]. The object is one of the many really faint dwarf galaxies within the Local Group, the gravitationally bound group of galaxies that contains the Milky Way and the Andromeda Galaxy, although it's located at the very edge of the group at a distance of 3.69 million light years (1.13 Mpc) from Earth [3]. Because the galaxy is both very small and very low in density, it basically looks like a collection of really faint stars spread over a region with a diameter between 5 and 6 times smaller than the diameter of the Moon [4].

What I personally found very interesting about the Sagittarius Dwarf Irregular Galaxy is that it is forming stars at a very slow rate. The rate of star formation is typically described as the mass of gas (in terms of the mass of the Sun) per year that is converted into stars, although keep in mind that stars take hundreds of thousands or millions of years to form, and also keep in mind that, when stars form, they come in a variety of sizes ranging from red dwarfs about 0.08 times the mass of the Sun to big blue stars about 60 times the mass of the Sun. So, for example, the star formation rate of the Milky Way is between 1.65 and 1.9 solar masses per year [5], which doesn't mean that one or two stars suddenly form every year but that maybe 1.65 to 1.9 million stars form every million years (although it's actually a mass of stars equal to 1.65 to 1.9 million times the mass of the Sun, but the simple way to think of it is as 1.65 to 1.9 million stars forming).

In comparison, the star formation rate for the Sagittarius Dwarf Irregular Galaxy is a really slow 0.0005 to 0.0021 solar masses per year [3], or in other words, 500 to 2100 stars form every million years. I study star formation in other galaxies, so I just want to really emphasize that this is incredibly slow. I normally expect dwarf galaxies to be forming stars much more rapidly or to be completely devoid of star formation. The fact that the Sagittarius Dwarf Irregular Galaxy just seems to be forming stars at an incredibly slow pace and that people can even pick out the individual stars that formed in the past few million years [6] is really mind blowing for me.

Having said that, I don't think other astronomers are as fascinated as I am by the incredibly slow star formation rate of the Sagittarius Dwarf Irregular Galaxy. Instead, I think most other astronomers are really interested in the fact that the stars in the galaxy appear to be composed almost entirely of hydrogen and helium. At first, this doesn't sound that remarkable because most people who understand a little bit of astronomy would already know that stars like the Sun are made out of mostly hydrogen gas with that hydrogen being fused into helium in the centers of those stars. However, the spectrum of the Sun as well as the spectra of other stars within our galaxy show that these objects actually contain much more than just hydrogen. Depending on the star, it may be possible to see carbon, nitrogen, oxygen, sodium, calcium, or iron, and in a few stars that have been studied more in depth like the Sun, it's possible to find a significant fraction of the rest of the periodic table. The Sun and most other stars in the Milky Way would have formed from gas ejected from supernovae and other dying stars, and those dying stars would have produced various elements heavier than helium through fusion either within their cores or when they were ejecting their outer gas layers into space, which is why the Sun and the other stars in our galaxy now contain small amounts of these heavier elements.

So, in contrast to stars in the Milky Way, the stars in the Sagittarius Dwarf Irregular Galaxy would have formed from gas that contains very few heavy elements. When astronomers look at the spectra of this galaxy's stars, they see just hydrogen and helium [6. 7]. One person declared that this galaxy may contain the stars with the purest mixture of hydrogen and helium in the Local Group [6], and I think she is right.

It's actually quite common to find that the stars and interstellar gas within dwarf galaxies contain fewer heavy elements than the stars and the interstellar gas in larger galaxies. I did not find any single reference that explained concisely why this is the case, but I can think of a couple of different reasons why this would happen. First, larger galaxies like spiral galaxies simply have more stars and interstellar gas and can also form more stars than dwarf galaxies. Since more stars are present in these larger galaxies, the stars when they die are going to eject more heavy elements into their interstellar mediums, and when that interstellar gas forms new stars, those stars will contain more heavy elements. Larger galaxies are also more likely to gravitationally interact with or absorb smaller galaxies (although the technical term is accrete and not absorb), and those interactions will lead to more stars forming and more stars exploding as supernovae, thus producing more heavier elements. Additionally, when supernovae explode in large galaxies, the ejected gas (which contains a lot of new heavy elements) will generally stay within those large galaxies because the galaxies exert a lot of gravitational force on the gas, but when supernovae explode in dwarf galaxies with much weaker gravitational fields, the gas from the supernovae is blown out of the galaxies, so the heavy elements from the supernovae are not used to form new stars within those dwarf galaxies.

So, the Sagittarius Dwarf Irregular Galaxy contains a lot of stars that are almost pure hydrogen and helium. It actually looks like the Sagittarius Dwarf Irregular Galaxy may be one of the best places not only within the Local Group but also in the known universe to look for these types of stars. Some of the physical processes within the stars in the Sagittarius Dwarf Irregular Galaxy are notably different from the stars that we see in the Sun and other stars in the Milky Way that contain heavy elements. Also, people who work on dwarf galaxies would tell you very excitedly that objects like the Sagittarius Dwarf Irregular Galaxy are really important to study because their stars are very similar to some of the first stars that formed when the universe was very young and contained virtually nothing but hydrogen and helium gas. After this, those people who work on dwarf galaxies would then tell you very excitedly that it's really important to give them funding and access to telescopes so that they can continue their research. Anyhow, given how exotic the stars are in the Sagittarius Dwarf Irregular Galaxy and how relatively close it is, it will be the prime focus of many research projects in the years ahead.


[1] Cesarsky, D. A. et al., Two new faint stellar systems discovered on ESO Schmidt plates., 1977, Astronomy & Astrophysics, 61, L31

[2] Longmore, A. J. et al., Two new dwarf galaxies in the Local Group: UKS 1927-177 and UKS 2323-326., 1978, Monthly Notices of the Royal Astronomical Society, 183, 97P

[3] Parto, Tahere et al., The Isaac Newton Telescope Monitoring Survey of Local Group Dwarf Galaxies. V. The Star Formation History of Sagittarius Dwarf Irregular Galaxy Derived from Long-period Variable Stars, 2023, Astrophysical Journal, 942, 33

[4] Higgs, C. R. et al., Solo dwarfs I: survey introduction and first results for the Sagittarius dwarf irregular galaxy, 2016, Monthly Notices of the Royal Astronomical Society, 458, 1678

[5] Evans, Neal J. et al., Slow Star Formation in the Milky Way: Theory Meets Observations, 2022, Astrophysical Journal Letters, 929, L18

[6] Garcia, Miriam, Massive stars in the Sagittarius Dwarf Irregular Galaxy, 2018, Monthly Notices of the Royal Astronomical Society, 474, L66

[7] Momany, Y. et al., The Sagittarius dwarf irregular galaxy: Metallicity and stellar populations, 2002, Astronomy & Astrophysics, 384, 393


Podcast and Website: George J. Bendo

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