# Students map Milky Way with dwarf stars

Isabel van Vledder and Dieuwertje van der Vlugt, 2 astronomy students from Leiden University, have mapped the entire Milky Way galaxy in dwarf stars for the first time. This result is the most comprehensive model ever for the distribution of these stars. The findings appear in a new paper in Monthly Notices of the Royal Astronomical Society.

### Structure of the Milky Way

The Milky Way, the galaxy we live in, consists of a prominent, relatively flat disc with closely spaced bright stars, and a halo, a sphere of stars with a much lower density around it. Astronomers assume that the halo is the remnant of the first galaxies that fused together to form our Galaxy.

### Counting stars

To find out exactly what the Milky Way looks like, astronomers have previously made maps using counts of the stars in the night sky. Rather than studying bright stars, Isabel and Dieuwertje used Hubble Space Telescope data from 274 dwarf stars, which were serendipitously observed by the orbiting observatory while it was looking for the most distant galaxies in the early Universe. The particular type of star they looked at were red dwarfs.

### Red dwarfs

Dwarf stars are undersized and often have too low a mass to burn hydrogen. As warm, rather than hot objects, they are best viewed with near-infrared cameras. Isabel comments: 'Astronomers believe that there are very many of these stars. That makes them really quite suitable for mapping the Galaxy even though they are so hard to find.'

### 3 models

To find the distribution of the dwarfs, Isabel and Dieuwertje used three density models that astronomers use to describe the flat disc and halo, both separately and combined. To calculate which model best describes the structure of the Milky Way, the students then applied the Markov Chain Monte Carlo method.

Dieuwertje describes how this works: 'You let a computer programme test all possible values of each parameter of your model. It then fixes the value which corresponds best with the data.'

### Perfect match

The model that includes both disk and halo was the perfect match. Isabel and Dieuwertje inferred the existence of 58 billion dwarf stars from the positions of the 274 M dwarfs in their sample. They were also able to accurately estimate the number of dwarfs in the halo, calculating a fraction of 7 per cent, higher than astronomers have previously found for the whole Milky Way.

### Future research

The students' results are important for future research with the European Space Agency’s Euclid Space Telescope, due for launch in 2020. Like Hubble, Euclid will image the whole sky in near-infrared. The students expect Euclid observations to yield an even more accurate picture of the Milky Way.

Isabel: ‘With our research, astronomers can now better assess whether they are dealing with a distant galaxy or a star in our own Galaxy.’

### Research during a bachelor's programme

Isabel en Dieuwertje carried out the research for their bachelor’s degree in Astronomy at Leiden University. They worked together with Leiden astronomers Benne Holwerda, Matthew Kenworthy and Rychard Bouwens.