The Strangest Stars in Outer Space

Outer space is pretty weird. Ask anyone who has ever heard of a black hole or seen the movie Interstellar, and they are bound to agree. But those points of light we see in the night sky, the stars we have been admiring for our whole lives, those seem more accessible to us. I mean, stars aren’t that weird, right?…

…Wrong.

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Artist rendering of a neutron star, which lies at the heart of a TŻO

Thorne-Żytkow Objects

Cue Thorne-Żytkow objects (TŻOs for short), the most bizarre stars that the universe could conjure. These objects are, in essence, a star inside of another star. That’s right, star inception – a theoretical stellar tootsie roll pop that you should definitely not try to lick.

TŻOs may be the strangest stars imaginable; that is, if they do in fact exist. Kip Thorne and Anna Żytkow first proposed these theoretical stars in the 1970’s as a thought experiment, a whimsical concept that proved interesting enough to explore. Since then, astronomers have been searching everywhere for TŻOs.

The formation of a TŻO is by no means impossible, but the two stars that constitute these astronomical anomalies would have to come together against all odds to form one. At the center of a TŻO sits a neutron star, and engulfing it, a great red giant. But how do these two completely different stars become one? Before we examine how neutron stars and red giants merge, let’s look at each star individually.

Neutron Stars

There are many types of stars in the universe, but neutron stars are some of the most peculiar objects floating through space. They are born when a massive star has a core so dense that it cannot support itself against its own gravity. The iron core of the star collapses and gives rise to either a black hole or a supernova. In the case of a supernova, the cataclysmic last gasps of a dying star, the outer layers of the star are ejected at million mile per hour speeds.

What’s left over is our neutron star, spinning hundreds of times per second in the eye of the storm. Neutron stars are typically less than 15 miles wide, but in theory they can weigh more than twice as much as the sun. These stellar oddities are among the densest objects in existence — a spoonful of neutron star stuff would weigh millions of tons.

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The Crab Nebula is a supernova remnant, cradling a neutron star at its center.

Red Giants

The other member of our stellar matryoshka doll is a red giant. Stars similar to our sun eventually become red giants as they reach the end of their life cycles. Once they use up all the hydrogen in their core, these sun-like stars resort to nuclear fusion (the physical process that powers the star) elsewhere within their interiors.

This causes the outer layers of the star to drastically expand 100 to 1,000 times its original size. Hence the name ‘giant.’ Since the energy produced within the star is now spread thin over a much greater area, the surface of the star becomes cooler and more red in color. Hence the name ‘red giant.’

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Red giants compared to the sun. For reference, an AU is a unit of distance representing the distance from Earth to the Sun.

Tying the Astronomical Knot

So how did two separate stars end up taking the plunge to become one? Turns out our neutron star and red giant knew one another back before things got complicated. To form a TŻO, long before either component begins the dying process, a very massive star and a sun-like star must exist in a binary system.

A binary star system is simply two stars that are gravitationally bound and orbit one another around a common center. Eventually, the heavier star will explode in a ferocious supernova and leave behind a neutron star. This happens much sooner than the sun-like star begins its giant phase — since larger stars burn up their hydrogen supply more quickly, they have a bad rap for living fast and dying young.

As the new neutron star and the sun-like star continue their stellar waltz, the latter starts to puff up into a red giant. After some complicated trip during their dance, the new red giant and the neutron star find themselves in an unstable orbit. Although astronomers aren’t sure why or when this happens, they know that this instability could cause the two components to spiral in towards one another until…BAM. The red giant swallows the neutron star whole. And there you have it, a cannibal star is born into the abyss.

Is There Anybody Out There?

So if all it takes is two clumsy balls of burning gas to form a TŻO, why aren’t they commonplace objects? For starters, usually the neutron star is kicked out of the system as a result of the shaky orbit instead of being sucked deeper into it. And even though it is estimated that a whopping 1% of all red giants are actually TŻOs in disguise, it is very hard to tell the two siblings apart.

At first glance, TŻOs should look just like normal red giants, except a little brighter. The only distinction between a TŻO and a true red giant is found in the spectral profile of the subject, which astronomers obtain by breaking up the light from their source using a spectrometer.

The spectrum of a star allows us to identify the elements within it by observing the varying intensity of its light at different wavelengths. It is expected that TŻOs should contain abnormally high concentrations of very rare elements, such as rubidium. Unless we can catch a TŻO at the moment of conception, studying their spectra is our only hope for distinguishing them from mainstream red giants.

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HV 2112 – a potential Thorne-Żytkow Object (note that this infrared image is in false color!)

Unfortunately, the process of sifting through stellar spectra is not a simple task. Only one star since the 1970’s has been labeled as a serious TŻO candidate. This potential TŻO, called HV 2112, has all the expected spectral signatures that indicate it is indeed the object astronomers hoped to find.

More analysis is necessary to confirm that HV 2112 is truly a Thorne-Żytkow object, but for now it is an exciting prospect. Identifying TŻOs could reshape how astronomers think about stellar evolution and explain the origin of many mysterious heavy elements in our universe.

image04Lauren Sgro is a PhD student in the Physics department at the University of Georgia. Her research focus is in astronomy, specifically debris disks around young stars that may tell us more about planetary formation. Despite the all-consuming nature of graduate school, she enjoys doing yoga and occasionally hiking up a mountain. You can’t reach her on Twitter, but you can email her at lauren.sgro25@uga.edu. More from Lauren Sgro.