Population (astronomy)

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In astronomy , a population or star population ( English stellar population ) is used to describe a subset of stars in a galaxy that have a similar metality (a similar age).

Original classification according to Baade

The classification goes back to Walter Baade (1944). It is useful for describing spiral galaxies such as the Milky Way , even if today's picture of these objects is much more complex. Although the point in time of its formation is a characteristic of a population, the Roman number (I, II or III) that denotes it does not correspond to the order in which it was formed, or it is exactly the opposite.

  • In the band of the Milky Way, the galactic disk, most of the stars belong to the population I . These are relatively young, stable shining stars, which move on approximate circular orbits around the galactic center , mostly in the spiral arms . Population I stars contain a relatively high proportion of heavy elements that were formed in earlier generations of stars and cause metal lines in the spectrum .
  • Population II stars can be identified by their lower metallicity (see e.g. BPS CS22892-052 ). Most of them are older than six billion years and are found, often in globular clusters , in the extensive galactic halo , the density of which increases towards the galactic center.

  • Shortly after the Big Bang , there were stars that, due to their large mass, quickly ended in pair instability supernovae , forming and hurling into space those metals that are already found in the oldest stars of Populations II. If there were also isolated low-mass stars, they will form the still hypothetical population III as cold dwarfs .

Finer division

Today, based on more precise measurement results, there are essentially five different populations.

Extreme Population I (spiral arm population)

The age of these stars is less than 100 million years. Such young stars are often found in spiral arms and irregular galaxies and there in diffuse nebulae , reflection nebulae , open star clusters and star associations .

Intermediate (older) population I.

Stars of this population, to which the Sun also belongs, have similar characteristics to those of the extreme population I except for their advanced age .

Disk population

Stars in this largest population have an average age. Most of the stars in our Milky Way galaxy belong to it, because they represent most of the galactic disk and the galactic center.

Intermediate (intermediate) population II

This is a smaller group that predominates mostly in the galactic center. This group includes above all the high-speed runners who move at speeds of over thirty kilometers per second perpendicular to the galactic plane .

Halo population (extreme population II)

These are stars over six billion years old. They are mostly found in globular clusters and elliptical galaxies . Very important members are the Unterdwarfs and the RR Lyrae stars .

HD 140283 and SM0313 as the oldest stars

HD 140283 is a star in the constellation Libra. It is one of the closest stars in Population II. With the help of the Hubble Space Telescope , the distance to it was determined to be 190 light years and its age - with an accuracy of ± 800 million years - estimated to be around 14.5 billion years.

Before that, the oldest star was discovered in 2014 (Sky Mapper Telescope, Siding Spring Observatory , Australian National University, and then Magellan ), SMSS J031300.36-670839.3 (SM0313) with an age of 13.6 billion years, so only formed around 100 to 200 million years after the Big Bang. It is only 6000 light years away in our galaxy. It belongs to population II, its age can be recognized by the fact that it contains hardly any iron (less than a ten millionth of the concentration in the sun) and very few other heavy elements, only hydrogen, helium, some carbon, lithium, magnesium and calcium.

Since it originated from supernova remnants of the first generation of stars (the previously unobserved Population III), astronomers had actually expected a higher iron content and take this as an indication that the first supernova explosions released relatively little energy and that the predominant part of the heavy elements in the resulting ones black holes disappeared. From the relatively high zinc content in HE 1327-2326 concluded Anna Frebel and colleagues in 2019, however, that the first stars found their end in a very asymmetric supernova explosions and have been distributed in this way, but relatively large amounts of heavier elements could.

Video

Individual evidence

  1. ↑ Sebastian Anthony, We´ve found the oldest star in the known universe , February 12, 2014.
  2. ↑ Rana Ezzeddine, Anna Frebel, I. Roederer, N. Tominaga, M. Ishigaki, K. Nomoto, J. Tumlinson, V. Placco, W. Aoki, Evidence for an Aspherical Population III Supernova Explosion Inferred from the Hyper-metal- poor Star HE 1327-2326, Astroph. J., Volume 876, No. 2, 2019, Arxiv

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