Artist’s conception of a red dwarf, the most common type of star in the Sun’s stellar neighbourhood, and in the universe. Although termed a red dwarf, the surface temperature of this star would give it an orange hue when viewed from close proximity. Image credit: NASA/Walt Feimer.Two astronomy students from Leiden University have mapped the entire Milky Way Galaxy in dwarf stars for the first time. They show that there are a total of 58 billion dwarf stars, of which seven percent reside in the outer regions of our galaxy. 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.
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.
Leiden Astronomy undergraduate students Isabel van Vledder (left) and Dieuwertje van der Vlugt (right). Image credit: Jose Visser (Leiden Observatory).To find out exactly what the Milky Way looks like, astronomers have previously made maps using counts of the stars in the night sky. Leiden Astronomy students Isabel van Vledder and Dieuwertje van der Vlugt used the same technique in their research. Rather than studying bright stars, the two students 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 of spectral class M.
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. Van Vledder 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.”
To find the distribution of the M dwarfs, van Vledder and van der Vlugt 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. Van der Vlugt describes how this works: “You let a computer program test all possible values of each parameter of your model. It then fixes the value which corresponds best with the data.”Fields observed by the Hubble Space Telescope where M-dwarf stars have been found, plotted on a map of the sky with galactic longitude and latitude. In each field, indicated by circles, only a few dwarf stars are identified. However, by combining them, the students could derive an accurate model of the Galaxy. Image credit: Leiden Observatory.The model that includes both disc and halo was the perfect match. From the positions of the 274 M dwarfs in their sample, van Vledder and van der Vlugt inferred the existence of 58 billion dwarf stars. They were also able to accurately estimate the number of dwarfs in the halo, calculating a fraction of seven percent, higher than astronomers have previously found for the whole Milky Way.
The results of the students 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. Van Vledder adds: “With our research, astronomers can now better assess whether they are dealing with a distant galaxy or a star in our own galaxy.” The students expect Euclid observations to yield an even more accurate picture of the Milky Way.
Van der Vlugt and van Vledder did 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.
In this NASA/ESA Hubble Space Telescope image we see the central Wolf-Rayet star known as Hen 2-427 — more commonly known as WR 124 — surrounded by the nebula M1-67. Both objects are found in the constellation of Sagitta some 15,000 light-years away. The hot clumps of gas ejected by the star into space are travelling at over 150,000 kilometres per hour.
Earth came early to the party in the evolving universe. According to a new theoretical study, when our solar system was born 4.6 billion years ago only eight percent of the potentially habitable planets that will ever form in the universe existed. And, the party won’t be over when the Sun burns out in another 6 billion years. The bulk of those planets — 92 percent — have yet to be born.
This glorious spiral galaxy is known as NGC 24, measures some 40,000 light-years across and lies about 25 million light-years away in the southern constellation of Sculptor. However, there may be more to this picture than first meets the eye: 80 percent of NGC 24’s mass is thought to be held within an invisible dark matter halo.
