Weirdest Stars in The Universe

The universe is full of amazing stuff, like colourful nebulae and stars that spin so fast that they pulse!

But today, let’s delve into the lesser known, odd stars in the universe starting with the smallest.

Brown Dwarfs

The babies of the universe…well not really.

The stars that are often talked about the gigantic stars, ones that quickly go out with a bang, and leave behind a supernova or black hole. But we rarely shine the light on the little ones.

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Artist’s impression of a brown dwarf. They don’t emit a lot of light, so they’re not very bright.

Brown dwarfs lie in the grey area between a planet and a fully functioning star. They are large enough that they can start some kinds of fusion, but not large enough to sustain it like a normal star. The mass cutoff between planets and brown dwarfs is about 13 x the mass of Jupiter, and the cutoff between dwarfs and regular stars is about 75 x the mass of Jupiter.

Alongside hydrogen, astrophysicists hypothesise that even the smallest of brown dwarfs may be able to fuse deuterium.

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A real image of a brown dwarf ( the orangey dot below the green) taken by SOAR/SAM

All stars start with gravitational contractions, which heat them up. In a normal star, these contractions heat the core until it ignites, releases enough energy to stop contractions, and starts fusion. In a brown dwarf, gravitational contractions aren’t large enough to heat the core to the right temperature, and eventually electrons in the atoms stop any more contractions.

The dwarf star stops trying to heat up, and just radiates the thermal energy away.

They’re cool stars, cool as in not hot. The coldest surface temperature brown dwarf observed is estimated to be room temperature.

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Another artist’s impression. Credit: R. Hurt/NASA

Magnetars

sun-earth-640-420-e1561152963905.jpgEarth’s magnetic field is about 0.0005 Tesla. That number seems small, but it’s strong enough to protect us from harmful solar radiation, channel away radioactive particles from Coronal Mass Ejection, and stop our atmosphere from being stripped by solar winds.

Now imagine a star that had a magnetic field 1 thousand billion times that.

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Magnetar SGR 1900+1 taken by SST

Magnetars are neutron stars with high magnetic field strength and rotational speeds. The field around them causes high energy gamma and X-ray bursts, a bit like a pulsar. These bursts are strong enough to disrupt the Earth’s atmosphere, which is what happened in 2004 when SGR 1806–20 released a gamma ray burst from the other side of the milky way.

Magnetars have a magnetic field of about 10¹² Tesla, which is strong enough to tear apart the atoms that make up your body within 1’000 km. But before that, you would be fried by the intense radiation.

It’s like getting snapped by Thanos, but more painfully.

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We don’t know how they form. One idea is that if the spin, magnetic field and temperature of a neutron star are high enough, they could set the perfect condition for a dynamo mechanism to form which increases the magnetic field by a large factor.

Wolf-Rayet Stars

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WR 124 taken by HST

Wolf-Rayet stars don’t look like any ordinary star: they’re wispy and fluffy!

Wolf-Rayet (abbreviated to WR) stars are incredibly massive and thus have a relatively short lifespan. Large stars like Wolf-Rayets use up their light elements (hydrogen and helium) in a very short time, so to sustain fusion, they fuse heavier elements like Oxygen and Silicon.

So what’s so special? Our sun is fairly small, but has enough power to strip away a minuscule amount mass outwards into space, usually hydrogen.

Wolf-Rayets are so massive that the energy they release is strong enough to strip a lot of the star’s own material away, which makes the star look fluffy and like it’s surrounded by a supernova.

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This is the Thor’s helmet nebula. The bright star in the helmet (near the centre) is WR 7. Taken by Adam Block/Mt. Lemmon SkyCentre/ U. Arizona

We don’t know how they are born, but the most likely possibility is that the Wolf-Rayet star was part of a binary system, where it fed off of the other star until it grew into the giant it is today.

At the very end of the star’s lifetime, the core is nothing but iron and so the star collapses and explodes into a large and beautiful supernova!

Zombie Stars

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G299, Type 1a Supernova. Credit: NASA/CXC/U.TEXAS

Type 1a supernovae are an extremely bright and large. They can be formed from a white dwarf accumulating mass from a companion star and exploding. This burst is quick and bright. They are most commonly talked about because the type 1a are all very similar in brightness, making them fantastic for measuring distances.

Another type of supernova is the type 1ax, which form similarly to the 1a, but release less mass at a much slower rate, and are very faint.

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Artist impression of a zombie star.

How do they form? Just like in type 1a supernovae: the white dwarf absorbs mass from its companion star but, for unknown reasons, only half of the dwarf explodes and leaves the other half intact, causing a type 1ax supernova instead. The star should be dead, but it isn’t.

The rest of the star is called a zombie star.

We know of only one zombie star, the remnant of the supernova SN 2012Z from the galaxy NGC 1309, formerly a binary system of a large star and a white dwarf.

supernova_2012z-e1561154047640.jpg
In 2005-06 Hubble saw the white dwarf (very dim here) and the companion star (blue). In 2013 the supernova was starting to explode. Taken by HST

Scientists believe that the white dwarf was engulfed by the larger star. They both didn’t combine completely, and the white dwarf started to accumulate matter from the larger companion until it went supernova. What was left was the zombie star.

 

 

Thorne-Żytkow Objects

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Artist rendering of a TZO.

A weird name for a weird star, or two stars actually. Thorne-Zytkow object (TZO) is a neutron star within a red giant, caused by both stars colliding into each other.

One way a TZO could form is by a main sequence star engulfing its companion neutron star when becoming a red giant. When a star like the sun burns up all its hydrogen and helium, its core contracts to get it to the right temperature to fuse helium.

The surface doesn’t contract, instead it expands. A companion neutron star may just so happen to be orbiting the star, and the surface expands past the neutron star’s orbit.

Here’s a quick doodle I made of the before expansion and after expansion. The core gets smaller but the outer layers expand:

The TZO doesn’t have to initially be part of a binary system. Another possibility is that the neutron star could have been ejected from a different binary system after its companion went supernova. The star would get hurdled out of the area and into a red giant.

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Artist rendering based on Paul G Beck’s work.

Once they’re together, their cores will very slowly combine and they will become a brand new neutron star, or a black hole if the combined mass is large enough.

Although we haven’t confirmed any TZOs, we have some strong candidates.

 

 

 

 

It was fun researching lesser known stars, and I hope you enjoyed learning about them. Are there any fascinating stars that you think I’ve missed out? Pop a comment below.

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