Could we have a star closer than Alpha Centauri hidden by dust?

What are the odds for a closer star than Alpha Centauri being hidden by a dust cloud? Like in Asimov’s Nemesis.
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I haven’t read Nemesis, so here’s my googled summary of the situation you’re referring to. In Nemesis, one of the characters discovers a previously unknown red dwarf star, only two light years from the Earth. This star is dubbed Nemesis, and, in the book, is on a path to mess with the orbit of the Earth such that the population of Earth needs to move.

Here’s the thing with both dust clouds and red dwarfs - they’re not totally invisible. A red dwarf star is fairly bright in the infrared, as dim as it might be in the optical - it only produces a little light that we can see, but far more as heat. If the star weren’t obscured with dust, it would be easily spotted by one of our all-sky surveys, as a weird bright source moving across the sky. Barnard’s Star, which is nearly 6 light years away, visibly moves through the sky - the gif below shows its motion over a 20 year period. There are a number of surveys hunting for moving objects, and though their primary target is usually asteroids, they’d still pick up on something like this.

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This motion means that the star can’t be coincidentally behind a convenient dust cloud that happens to be along the line of sight between us and that star. It would quickly move out from behind the blocking cloud. (As a further point, our solar system isn’t in an area of the galaxy that’s particularly full of dust clouds, so it’s unlikely it would have anything to hide behind in the first place.

In any case, this means that if we’re going to have a star buried in dust and therefore invisible, it’s going to have to be carrying the dust with it. The easiest way for that to happen is if it never got rid of the dust cloud that it formed in - however, red dwarf stars are usually extremely old, and would have had time to burn away the dust that surrounds them, so this is also unlikely.

Even if it did somehow have a cloud of dust surrounding it, dust glows, if you’re looking at the right wavelength. The image below is the glow of carbon monoxide in our galaxy as taken by the Planck space telescope. Carbon monoxide only exists in very dense clouds of gas, the sort of dense cloud you need if you want to form new stars - any less dense and the molecule will be destroyed. We should be able to spot anything that’s very dense through one of these maps, if we hadn’t noticed it through its motion.

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So I would say that the odds are pretty low that there’s a surprise red dwarf anywhere near our solar system. The odds are much higher that we’ll run into danger from within our own solar system - asteroids which cross the orbit of the earth are pretty common.

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What kinds of dwarf star are there?

What different types of dwarf star are there and could you mention a bit about each? e g. Some brown dwarf stars have liquid iron raining down on them.

“Dwarf” was originally a term used to distinguish between the two kinds of red stars in the universe - very massive, and very small. These were termed “red giants” and “red dwarfs”. The dwarf terminology gradually expanded to mean “not giant” stars of any colour, and the line between “giant” and “dwarf is somewhat poorly defined; the Sun is technically a "yellow dwarf” star.

What most people think of when they hear “dwarf star” are brown dwarf, red dwarf, and white dwarf stars. There are also a few theoretical kinds of dwarf stars, which is where black dwarfs fall. These stars are all classified based upon their colour, although confusingly these are not usually the colors they would appear to our eyes. (Brown dwarfs, for instance, would appear a deep pink - see above for 3 brown dwarfs as they would appear to us.)

Yellow and red dwarf stars are normal stars - they burn hydrogen in their cores and live on the main sequence of stellar lifetimes. Red dwarfs are smaller than our sun, only getting up to 50% the size of our sun. As a result, their surfaces are cooler, hence the colour shift towards the red. They don’t consume their hydrogen as quickly as our sun does, so even though they’re less massive and thus have less hydrogen, they still live for a much longer time than our sun will. Because red dwarfs require less matter to create, they are the easiest to make. Red dwarfs are therefore the most abundant type of star in the galaxy - our nearest stellar neighbor is a red dwarf.

Brown dwarfs are failed stars. They’re essentially massive Jupiters - large collections of gas that are not massive enough to create the pressures required to start burning hydrogen into helium. These dwarfs can be pretty cold; there was one found not too long ago that was only as warm as a cup of coffee. A brown dwarf can’t do anything except sit there and slowly radiate away its heat - it won’t ever become a fully fledged star. The iron rain you refer to was the conclusion of a study from 2006; evidence was found that at the temperatures of the star they were looking at, the iron they detected in its atmosphere should be forming liquid droplets and raining down towards the surface of the star. Further studies have found evidence for massive, Jupiter-style storms in the atmospheres of these stars. The behavior of the metals and other elements in a brown dwarf’s atmosphere will depend strongly on the temperature of the star in question. Since “brown dwarf” is a rather broad term, some of these stars will be too cold for iron rain, and some will be too warm. Of course, the presence or absence of a particular element will depend on the gas the dwarf formed out of, since the brown dwarf is not building any new elements itself.

White dwarfs are the most exciting to make. They are what is left over after a main sequence star (like our Sun) dies. The star will have gone through the red giant phase, and then shrugs off its less dense outer layers into a planetary nebula. At the end, all that is left is a hot, dense core of what was once the centre of the star in a volume about that of the Earth. They are so dense that the pressure provided by the electrons of the atoms within the star pushing against each other is what keeps them from becoming any smaller, and so hot they glow white just from trapped heat. This is the end-point of our Sun.

The black dwarf - still a theoretical object - is the name we would give to a white dwarf star which had managed to completely lose all of its heat, effectively going completely out. The length of time it takes for a white dwarf star to lose all of its heat is longer than the length of time the Universe has been around, so we don’t expect to see many of these around.

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What would happen if the sun split in half?

This is not possible due to the force of inertia and gravitational force, along with nuclear fusion, but: What would happen if the sun split into 2 ½ – Sized suns?
DG CVn (right) is about one-third the size of our sun (left).  Image credit NASA

DG CVn (right) is about one-third the size of our sun (left). Image credit NASA

Let’s assume that we’re just replacing our sun with a pair of stars which are in a stable orbit around each other, each of which is half the size of the sun, because if our replacement stars are in unstable orbits, this will get super complicated really fast. Even with this simplifying assumption, our solar system would be a very different place. 

A star’s energy output is strongly related to how much mass it has, but it’s not a one to one relation. If you cut the sun’s mass in half, you reduce its brightness by 90%. Gravity doesn’t have as much mass to work with, so the force of gravity can’t crush the star down as much, and the star can’t reach the same pressures and temperatures in its center as a more massive star can. These half-stars are at the upper edge of what’s considered a red dwarf star; they can still burn hydrogen in their cores, but at a much slower rate than our sun. Slower energy production gives you a dimmer star, and a dimmer star means a cooler, redder star.

This means, therefore, that if you cut the sun’s mass in half, you go from a star that looks yellow-white, and has a surface temperature of 5800 Kelvin (9980 F, or 5527 C) to something that will look distinctly orange, tipping towards the red end of orange.  Each of these half-suns would have a surface temperature of only 3700K (6200 F or 3426 C), a drop in surface temperature of 40%.

Our replacement set of stars has a combined energy output of 20% what our sun’s brightness is. As a result of this drop, the other thing that will definitely change about our solar system is the distance from the stars where liquid water can pool on the surface of the planet. With 20% the output of our current sun, Earth would be way too far away from these stars - all the water on our planet would freeze. In fact, the habitable zone would be almost exactly where Mercury’s orbit is now. Mercury’s orbit goes between 0.3 and 0.46 AU (AU being the distance between the earth and the sun). The habitable zone would range between almost these same numbers: 0.32 AU, if the planet was getting light from only one star, up to 0.44 AU, if it was getting sunlight from both of them.

Mercury is really close to the sun. So if we put our habitable planet in the right spot to have life, it will also have to be really close to the two half-suns. But then you start getting into really hairy territory, because any time you have three objects interacting with each other, as you would with a close pair of stars and a nearby, light planet, the orbits can get pretty funky - this is called the 3 body problem. It’s not nearly as nice as having one massive object in the centre for things to calmly orbit around. There are windows of stability, where the stars and the planet can orbit relatively calmly for a long enough time to develop life. The positions of these windows depends entirely on the exact configurations of the orbit of the two stars in question. If the planet is not in a window of stability, one of your three objects can wind up getting tossed out of the solar system, which would be particularly bad for the planet you’re trying to grow things on.

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