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.
LISA Pathfinder, a mission led by the European Space Agency with contributions from NASA, has successfully tested key technology needed to build a space-based observatory for detecting gravitational waves. These tiny ripples in the fabric of space, predicted by Albert Einstein a century ago, were first seen last year by the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO).
This NASA/ESA Hubble Space Telescope image reveals NGC 1222, a galaxy with a rather eventful story to tell. NGC 1222 has been described as a peculiar example of a so-called lenticular galaxy, but one that exhibits very recent star formation on a huge scale — an event known as a starburst — due to having recently consumed two dwarf galaxies.
NGC 4639 is a beautiful example of a type of galaxy known as a barred spiral. It lies over 70 million light-years away in the constellation of Virgo and is one of about 1500 galaxies that make up the Virgo Cluster. But NGC 4639 also conceals a dark secret in its core — a massive black hole that is consuming the surrounding gas and known as an active galactic nucleus (AGN).
