This is an artist's concept of a red dwarf star undergoing a powerful eruption, called a stellar flare. A hypothetical planet is in the foreground. Credit: NASA/ESA/G. Bacon (STScI)
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For a long time, astronomers have known that stars often have troubled childhoods. They suffer from frequent and violent flares. But eventually, as they settle onto the main sequence, stars grow out of their destructive ways, which is thankful for us since large flares could do some serious damage to our biosphere. A new study confirms expectations that some stars never outgrow their roguish ways and that the smallest stars can be prone to the most frequent flares.
The study uses data from the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) survey conducted by the Hubble Space Telescope. This survey was conducted over a seven day period in 2006 and originally designed to search for transiting planets by repeatedly imaging over 200,000 stars for sings of transits. However, since the exploration contained so many red dwarf stars, the smallest and most common stars in the universe, a team led by Rachel Osten of the Space Telescope Science Institute was able to use it to constrain the rate of flares on these diminutive stars.
The team eventually discovered 100 stellar flares, some of which increased the overall brightness of their parent star by as much as 10%. In general, most flares were short, lasting on average a mere 15 minutes. Some stars flared multiple times. These flares weren’t limited to simply young stars, but also, highly evolved stars, including several variable stars which appeared to flare more often.
“We discovered that variable stars are about a thousand times more likely to flare than non-variable stars,” Adam Kowalski, another team member, says. “The variable stars are rotating fast, which may mean they are in rapidly orbiting binary systems. If the stars possess large star spots, dark regions on a star’s surface, that will cause the star’s light to vary when the spots rotate in and out of view. Star spots are produced when magnetic field lines poke through the surface. So, if there are big spots, there is a large area covered by strong magnetic fields, and we found that those stars had more flares.”
Part of the reason that dwarf stars are though to flare more comes from the fact that they have deep convection zones (shown by their lack of lithium in the photosphere which is destroyed by convection which drags it to depths hot enough to destroy it). This bulk movement of ionized particles creates a dynamo and strong magnetic fields on the star. When these fields become especially tangled, they can snap and spontaneously reform in a lower energy state. The energy lost is dumped into the stars outer layers, heating them with tremendous amounts of energy and releasing large amounts of ultraviolet, X-ray, and even gamma radiation as well as charged particles. In more extreme circumstances, the fields don’t immediately reform but swing outwards as they unwind themselves, dragging large amounts of the star with it, and flinging it outwards in a coronal mass ejection (CME).
One of the results of the enhanced magnetic activity is a larger number and size of sunspots. According to Osten, “Sunspots cover less than 1 percent of the Sun’s surface, while red dwarfs can have star spots that cover half of their surfaces.”
Artist's depiction of the Morgan-Keenan spectral diagram, showing how stars differ in colors as well as size. Credit: Wikipedia Commons
A star is a star, right? Sure there are some difference in terms of color when you look up at the night sky. But they are all basically the same, big balls of gas burning up to billions of light years away, right? Well, not exactly. In truth, stars are about as diverse as anything else in our Universe, falling into one of many different classifications based on its defining characteristics.
All in all, there are many different types of stars, ranging from tiny brown dwarfs to red and blue supergiants. There are even more bizarre kinds of stars, like neutron stars and Wolf-Rayet stars. And as our exploration of the Universe continues, we continue to learn things about stars that force us to expand on the way we think of them. Let’s take a look at all the different types of stars there are.
Protostar:
A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy release by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.
Size chart showing our Sun (far left) compared to larger stars. Credit: earthspacecircle.blogspot.ca
T Tauri Star:
A T Tauri star is stage in a star’s formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they’re about the same temperature but brighter because they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.
Main Sequence Star:
The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they’re all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.
A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.
The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun, or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun.
Red Giant Star:
When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.
White Dwarf Star:
When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity. A white dwarf shines because it was a hot star once, but there’s no fusion reactions happening any more. A white dwarf will just cool down until it becomes the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.
Red Dwarf Star:
Red dwarf stars are the most common kind of stars in the Universe. These are main sequence stars but they have such low mass that they’re much cooler than stars like our Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.
Neutron Stars:
If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn’t form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.
Supergiant Stars:
The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.
As you can see, stars come in many sizes, colors and varieties. Knowing what accounts for this, and what their various life stages look like, are all important when it comes to understanding our Universe. It also helps when it comes to our ongoing efforts to explore our local stellar neighborhood, not to mention in the hunt for extra-terrestrial life!
