Red dwarfs: The most important stars in the Universe

The smallest stars in the galaxy are the red dwarfs.  Recent research suggests they are more common than previously suspected. Tiny, cool and dim compared to the Sun, they may seem unimportant. But consider this: red dwarfs not only outnumber every other type of star in the Universe but will outlive every other type of star.

Image of Red Dwarf companion

Artist's impression of the red dwarf star CHRX 73 A and its companion object CHRX 73 B. The companion object is around 12 Jupiter masses, and may either be a planet or a brown dwarf. (Image credit: NASA, ESA and G. Bacon (STScI))

In Astronomy-speak, red dwarfs are low-mass stars; our own Sun is 2.5 times as heavy as even the biggest red dwarf. Compared to the Sun, the pressure deep in a red dwarf’s core is low (a consequence of the lower mass). Lower pressure in the core means the temperature there is lower too, hence nuclear fusion reactions run at a slower rate. Nuclear fusion, the process by which  hydrogen atoms combine into helium atoms releasing energy as a by-product,  is the engine of the stars. Consequently red dwarf stars are faint, at the very most blazing only a tenth as bright as the Sun and sometimes glimmering only 1/1 0000th as bright as the Sun.

All main sequence stars are living off their savings, generating energy by burning the hydrogen they formed from in some ancient stellar nursery. When the hydrogen starts to run out (and the helium starts piling up), the star is in decline and its final fate as either a white dwarf or supernova remnant (exactly which depends on its mass) is in sight. Red dwarfs are stellar misers, gradually spending their hydrogen to provide the energy needed to keep the light on, making their tiny stockpile of fuel last as long as feasible. The smaller the red dwarf, the slower it burns.

Just how long a red dwarf can last is astonishing, in fact, red dwarfs are estimated to have potential lifetimes longer than the 13.7 billion or so years the Universe has existed. In other words, no red dwarf has yet died, the Universe isn’t old enough! The Sun is expected to exist as a main sequence star for some 10 billion years, but a red dwarf one tenth its size ought to shine on for a thousand times longer, ten trillion years.

In our galaxy, the Milky Way, red dwarfs are the most common type of star. In the vicinity of the Sun, 80% of the stars are red dwarfs. So where are they? In the Northern Hemisphere, look towards Leo, home of Wolf 359, a red dwarf a tenth the mass of the Sun, and only 7.8 light years away. Observers in the Southern Hemisphere can look to Alpha Centauri, actually a multi-star system one of which is celebrated red dwarf Proxima Centauri. Neither Wolf 359 nor Proxima is visible to the naked eye. No red dwarf can be seen from Earth with the unaided eye. We can only gaze at the minority of large and bright stars. If we could see all the red dwarf stars around us, the night sky would be a much more cluttered place!

In the wider Universe, it turns out that red dwarfs are even more common than previously thought.  Being small and dim, red dwarfs are difficult to detect in galaxies beyond the Milky Way.  This meant astronomers were unsure of how many stars in the Universe were red dwarfs.  However this has changed. Using the sensitive instruments at the Keck Observatory in Hawaii, astronomers from the Harvard-Smithsonian Center for Astrophysics (CfA) have found and counted red dwarfs in the cores of eight elliptical galaxies, located between about 50 million and 300 million light-years away. The biggest galaxies in the universe are elliptical galaxies like these. The largest of these were thought to made of over a trillion stars, compared to 400 billion in our Milky Way.

image of elliptical galaxy

Elliptical galaxies like the one in this artist's conception are the biggest galaxies in the Universe. The largest of these hold over one trillion stars according to astronomical census takers, compared to 400 billion in our Milky Way. However, new research shows that elliptical galaxies actually hold five to ten times as many stars as previously believed. (Image credit: David A. Aguilar (CfA))

The CfA researchers found that red dwarfs were much more common in these elliptical galaxies than previously expected, implying that elliptical galaxies actually possess five to ten times as many stars as thought before. After a spot of number crunching, this means that the total number of stars in the whole universe is probably three times bigger than we thought .

Discoveries like this make astronomy a joy for me. You go to bed and when you wake up the Universe has grown over night!

Image of red dwarf planet

Red dwarf planet: This artist’s impression shows the super-Earth exoplanet orbiting the nearby star GJ 1214. This is the first super-Earth to have its atmosphere analysed. Only 40 light-years away from Earth, the planet, GJ1212b has a mass about six times that of the Earth. The planet appears to be surrounded by an atmosphere that is either mainly steam or else blanketed by thick clouds or hazes. (Image credit: ESO/L. Calçada)

There are deep consequences to this exciting finding. It shows that not all galaxies are equal, clearly if red dwarf stars are more common in elliptical galaxies than spirals like the Milky Way then this indicates there is some significant difference in the conditions in the various forms of galaxies. Also, this finding may suggest that some of the dark matter, that enigmatic unseen something whose gravity stops galaxies flying to pieces could in fact be red dwarf stars (note if this is true this does not solve the mystery of dark matter, the larger part of it would still be unidentified).

More red dwarf stars will mean many more Earth-like (in the widest possible sense) planets, some potential homes for alien life. The habitability of red dwarf planetary systems is much debated by those interested in extraterrestrial life. Planets with liquid water will be in tight, star-hugging orbits with years measured in a handful of terrestrial days. This will lead to the planets being tidally locked to the star, permanently keeping one face in an eternal day, the other in endless night. The weather and environment on such a world would be very alien to us Earth people.

However, any life that arises on a red dwarf world is in it for the long haul. A G-type star has but a brief window for life to flourish on its planets, before it expands to a red giant in a cosmic eye blink, then spending perhaps a quadrillion years as a cooling white dwarf. In contrast, a red dwarf will shine steadily but faintly on its children across a lifetime perhaps a thousand times that of the Sun. Eventually there will be a time when red dwarfs are the only living stars in the Universe, all the others which gleam so bright today will be dead stellar husks. If thinking beings can arise on the planets of red dwarfs, one day the Universe will belong to them.