Astronomy Essentials

What’s a red dwarf? Only the most abundant Milky Way star

Closeup of gray rocky planet with another distant planet and small, faraway reddish star.
View larger. | This artist’s concept depicts a distant red dwarf star with possible planets orbiting it. Red dwarf stars are the most common type of star in the Milky Way galaxy. Image via NASA/ ESA/ CSA/ Joseph Olmsted (STScI)/ Webb Space Telescope.

What’s a red dwarf star?

Red dwarf stars are extremely common, at least in our Milky Way galaxy. They make up some 60 to 70% of all stars in our galactic home. In fact, the closest star to Earth, Proxima Centauri, is a red dwarf. And yet, you can’t see it with your eye alone – or any other red dwarf – because these stars are too dim. Red dwarf stars’ main characteristics are that they’re small, cool and live a long time. And, of course, they have a distinct red color.

The famous Hertzsprung-Russell diagram (or H-R diagram for short) lets you visualize where stars rank compared to other stars and throughout a star’s lifetime. Red dwarfs earn the classification of Type M. The red color is a sign of their low temperature. Cooler stars in the universe radiate light toward the red, long-wavelength end of the spectrum. Meanwhile, the hottest stars radiate toward the blue, shorter-wavelength end and shine blue or blue-white. In the same way, a poker put into a fire will start glowing with a dim red color. It will then glow orange, yellow and finally white as its temperature increases.

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Chart showing long swaths of groups of stars of varying colors and sizes.
View larger. | This is the famous Hertzsprung-Russell diagram, which shows the luminosities of stars. Our sun is an average star, toward the center of the diagram. Red dwarf stars are toward the lower right on the chart. Proxima Centauri, the nearest star to Earth, is a red dwarf. Image via ESO.

Characteristics of a red dwarf star

Temperature: The surface temperature of red dwarf stars ranges from 2,000 to 3,500 degrees Kelvin (3100-5800 F or 1700-3200 C). Our sun is much hotter, at 5,500 degrees (9400 F or 5200 C), and it glows yellow as a result. A lower temperature also means lower luminosity: The largest, most luminous red dwarfs are only about 10% of the brightness of the sun. The smallest, dimmest ones are only around 0.075% our star’s brightness.

Size: The size of the smallest red dwarf stars is about 9% the radius of the sun. The largest red dwarf known, DH Tauri, has 1.26 times the radius of our sun. So, if the largest red dwarf can be bigger than our sun, then is our star called a dwarf star? And, in fact, astronomers do consider the sun a yellow dwarf star on the main sequence of the H-R diagram. While some ancient cultures worshipped the sun as the most powerful thing in the universe, in reality it’s a small and insignificant star. From just a few light-years away, extraterrestrial travelers might not even give it a second glance!

Lifespan: Red dwarfs are incredibly long-lived. They can live for tens of billions up to trillions of years. In other words many times the current age of the universe! But why is this?

The evolution of a red dwarf star

The key to understanding red dwarfs’ incredible longevity is their mass. Nuclear fusion is at the heart of every star, converting hydrogen into helium and producing heat, light and electromagnetic radiation. This nuclear fusion works at a rate governed by the mass of the star. The more massive the star, the greater the temperatures and pressures at its core, and the faster the fusion process proceeds. And vice versa. Red dwarfs typically have less than half of the mass of our sun. So, hydrogen is converted into helium at a slower rate. The end result is that red dwarfs evolve in slow motion compared to more massive stars.

However, red dwarfs, just like our sun, will one day exhaust their supply of hydrogen. In the case of the sun, this will happen when our star is around 8 to 10 billion years old; in other words, around 5 billion years from now. But because of the slow-motion fusion processes at the core of a red dwarf, this stage won’t arrive until the star is trillions of years old!

In the case of stars like our sun, the exhaustion of its hydrogen supply results in the star slowly inflating into a red giant star, many times its original diameter. But with red dwarfs, this doesn’t happen. Why? Once a star like our sun has exhausted its hydrogen, it starts to fuse helium, which triggers the inflation. However, red dwarfs don’t have enough mass to start fusing helium. Instead, the red dwarf stars bypass the red giant phase. Instead, they’ll slowly shrink and cool at the end their lives, becoming white dwarf stars. This is also our sun’s destiny after its red giant phase.

Exoplanets around red dwarfs

Astronomers have discovered planets orbiting red dwarf stars. What would it be like to live on one?

To an observer on a planet orbiting a red dwarf star, the star would appear to be much larger than the sun in our sky. But why, if red dwarfs are so much smaller than the sun? So far, the planets astronomers have discovered orbiting red dwarfs orbit much closer to their star. The red dwarf would therefore appear much larger than the sun does in our own sky.

The color of the red dwarf star would also be very different from our sun. Our sun emits most of its light in the yellow and green wavelengths of the spectrum, which is why it appears yellow to us. Red dwarf stars, on the other hand, emit most of their light in the red and infrared wavelengths. Thus, they would appear orange-red in the sky. The longer-wavelength light would also mean that the planet’s surface illumination would be far less. Everything on the planet would be dimmer and cast in red tones. Scientists think daytime on planets orbiting red dwarfs would never get any brighter than a sunset does on Earth.

Surveys have discovered that most of the planets orbiting red dwarfs are either comparable in size to Earth or are super-Earths. Scientists estimate that gas giant planets, like Jupiter, Uranus and Neptune, make up just one in 40 planets orbiting red dwarfs. In addition, computer simulations of red dwarf exoplanets indicate that at least 90% of them are at least 10% water by volume, meaning they may have global oceans.

Life on red dwarf exoplanets?

Because a red dwarf is much lower in temperature than stars like our sun, the red dwarf’s habitable zone is much closer to the star than in a planetary system like ours. Therefore, even though the planets we’ve found are closer to their red dwarf star, they might still be in the habitable zone.

However, before we get too excited about this possibility, there is one problem. Red dwarf stars are known for their violent solar flares. These flares can be up to a thousand times more powerful than the largest flares from our sun. Red-dwarf flares can emit intense radiation that can strip away the atmospheres of planets and make them uninhabitable. However, studies have shown that these flares may not be as destructive as previously thought. Flares tend to occur at high latitudes on the surface of a red dwarf, which means they may not strike planets that are orbiting closer to the star.

Solar flares on red dwarf stars are caused by magnetic activity. That’s the same thing that causes them on our sun. Red dwarf stars have very strong magnetic fields, which can become tangled and release huge amounts of energy in the form of a flare. Flares on red dwarf stars can last for hours or even days. And, they can release enough energy to power the entire Earth for centuries!

Is there a reasonable chance, therefore, that Earth-sized exoplanets orbiting red dwarfs, with their apparent abundance of water, might be hosts for life? Let’s look at one example of such a planetary system. This is a system that has astronomers and astrobiologists excited with its possibilities for life: the TRAPPIST-1 system.

The TRAPPIST-1 system

The TRAPPIST-1 system lies about 40 light-years from Earth. It’s home to seven Earth-sized planets, all orbiting the ultracool red dwarf. The planets are all quite close to their star, with orbital periods ranging from 1.5 to 19 Earth days.

Three of the TRAPPIST-1 planets are in the star’s habitable zone. This location makes them some of the most promising candidates for life outside our solar system that we’ve yet found.

When astronomers give names to exoplanets, they do so by designating each planet a letter, where “a” is the star itself, “b” is the planet orbiting closest to the star, “c” the next most distant, and so on. Observations with the James Webb Space Telescope have shown that:

  • The surface temperature of TRAPPIST-1b is around 230 degrees Celsius (450 degrees Fahrenheit), making it too hot for liquid water to exist on its surface.
  •  TRAPPIST-1c likely has a very thin atmosphere, or no atmosphere at all.
Oblique views of two solar systems with a wide green band in each.
The Trappist-1 planetary system’s habitable zone, compared to our solar system’s. Planets are not to scale. Image via NASA/ Wikipedia (public domain).

A chance for life near TRAPPIST-1?

However, intense and constant magneto-solar activity on TRAPPIST-1 is interfering with the Webb’s ability to obtain reliable spectra of the star. And from these spectra, scientists tease out the spectra of the planets’ atmospheres. So the spectra will probably need future observations and reanalysis. But, at the moment, it looks as if neither TRAPPIST-b or TRAPPIST-c is a likely candidate for life. Webb observations of the remaining planets, TRAPPIST-d to TRAPPIST-h, are still to come. Then, perhaps, we’ll have a profile of the whole system and understand a little more about red dwarfs, their activity and the effect they have on their planets.

Of course, all this does not mean that the TRAPPIST-1 system is typical of red dwarf planetary systems. It’s a mistake to draw general conclusions from what Webb discovered about its planets. We need to observe other planets orbiting these small, red stars. For example, astronomers have confirmed the existence of three exoplanets in the Gliese 581 system. This red dwarf is the oldest, least active M-type star currently known. Unfortunately, the three planets seem to orbit closer to their star than the inner edge of the habitable zone, so are likely too hot to support life.

The closest star to the sun, Proxima Centauri, is also a red dwarf. It has two confirmed Earth-sized exoplanets, one of which orbits in the habitable zone. However, little is known about this planet at the moment.

Understanding red dwarfs

Understanding red dwarf planetary systems is important, both for studying stellar evolution and in the hunt for extraterrestrial life. If it is true that exoplanets orbiting red dwarfs are never capable of supporting life, then at least 60% of the stars in our galaxy have lifeless systems. And that is significant.

Astronomers will continue to study these small, red, abundant stars in order to understand exactly how they behave and whether they’re capable of giving birth to exoplanets where we might find life. Red dwarfs are the obvious place to search for life-bearing planets because they’re cool, and therefore their habitable zones are close-in. That means that the length of a year for these planets – the time it takes to orbit their star – is much shorter. So, we can make repeated observations of them over a comparatively short time period as they transit across the face of the red dwarf.

Red dwarfs are an interesting area of study for astronomers and planetary science. They seem to be our best bet for finding planets with life. However, there is still a lot about them which we don’t understand. But with incredible tools such as the Webb and the upcoming planet-finding instruments, our knowledge of red dwarfs can only increase.

Bottom line: A red dwarf star is the smallest and coolest type of star known. They’re also extremely common, making up around 60 to 70% of the stars in the Milky Way.

December 3, 2023
Astronomy Essentials

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