The TZO: Light and fluffy outside, dense and crunchy inside

September 16, 2014 5:39 PM

Binary stars are common in our galaxy. In fact, singleton star systems like ours make up only 15% of the systems in the Milky Way. In the 1970s, astronomers Kip Thorne and Anna Żytkow, imagined what might happen if a neutron star in a binary system merged with its partner, a red supergiant. Recently, a real example of this strange star-within-a-star, known as a Thorne–Żytkow object (TZO), appears to have been spotted. (Preprint.)
posted by CheeseDigestsAll (21 comments total)

...analyzing its spectra for an over-abundance of lithium and other heavy metals.

This is why I hate reading popularizations written by scientifically illiterate reporters. Since when does lithium count as a "heavy metal"?
posted by Chocolate Pickle at 6:10 PM on September 16, 2014

Less commonly, any metal with a potential negative health effect or environmental impact may be termed a heavy metal, such as cobalt, chromium, lithium and even iron.

According to the International Union of Pure and Applied Chemistry or IUPAC, the term "heavy metal" may be a " meaningless term " because there is no standardized definition for a heavy metal.

posted by spikeleemajortomdickandharryconnickjrmints at 6:14 PM on September 16, 2014

Since when does lithium count as a "heavy metal"?

I agree. Nirvana were grunge.
posted by hal9k at 6:14 PM on September 16, 2014 [7 favorites]

Astronomers count elements as: hydrogen, helium, metals. Makes the periodic table much easier.
posted by physicsmatt at 6:19 PM on September 16, 2014 [13 favorites]

Since when does lithium count as a "heavy metal"?

In astronomy, there's Hydrogen, Helium and Metals.

Yeah, it's confusing, but really, the important things are the rations between H, He and Everything Else. A star's metallicity is the percentage of it's mass that isn't hydrogen or helium.

So, yeah, Li is a metal.
posted by eriko at 6:32 PM on September 16, 2014

So Neon is a metal?
posted by Chocolate Pickle at 6:35 PM on September 16, 2014

If you had asked me if Neo was "metal" I would have said "no", but I learned something today.
posted by maxwelton at 6:38 PM on September 16, 2014

Chocolate Pickle:

"So evidently Neon is a metal?"

It's not just metal, it's Christian Metal.
posted by Hairy Lobster at 6:40 PM on September 16, 2014

So, yeah, Li is a metal.

I think the objection is more to "heavy" than metal. Cuz I'd think of sodium and potassium as metals, but it's odd to call the third lightest element a heavy anything.
posted by LionIndex at 7:05 PM on September 16, 2014 [1 favorite]

oh man words mean different things in different contexts who knew
posted by griphus at 7:05 PM on September 16, 2014 [5 favorites]

Enough with the word derail already, this is a freaking awesome discovery, a star within a star, as predicted by theory. Fantastic astronomy and science in action.
posted by marienbad at 7:16 PM on September 16, 2014 [3 favorites]

The grapevine says that Kip Thorne is off consulting on realistic special effects involving black holes for an upcoming Hollywood blockbuster ("Interstellar"), but note that Anna Zytkow is a co-author on both the papers (abstract 1, abstract 2).

They're cool abstracts, but the evidence is by no means definitive yet. Possible, certainly. Probable? Maybe, I guess. Just read the two abstracts and decide for yourself.
posted by RedOrGreen at 7:50 PM on September 16, 2014

If you had asked me if Neo was "metal" I would have said "no", but I learned something today.

WE ARE WYLD STALLYNS
posted by Sebmojo at 7:51 PM on September 16, 2014

I don't understand why the neutron star does not disintegrate because of the gravitational differential between it and the red giant.
posted by Renoroc at 8:02 PM on September 16, 2014

A neutron star's surface gravity is hundreds of billions of G's, far too great to be disrupted by tidal effects.
posted by Chocolate Pickle at 8:27 PM on September 16, 2014 [1 favorite]

So I was pretty sure that the neutron star was in no danger of being ripped apart, but it is interesting to see how the numbers work out. Short answer: the neutron star is fine. The supergiant not so much.

Tidal effects can rip apart smaller objects as they approach larger ones. One way to estimate this would be the Roche Limit. If the tidal acceleration (the difference in gravitational acceleration from the side of the smaller object close to the larger object to the far side of the smaller object) is larger than the acceleration due to the gravity of the smaller object at it's own surface, then it is reasonable to expect that the smaller object will be torn apart. Basically, when this happens, objects sitting on the near surface of the smaller object would be pulled in towards the larger body faster than the rest of the smaller object, and the small object (usually a moon) will rip itself apart over time.

The supergiant here might have a mass of 15 M_sol, and a radius of 1000 solar radii. So a mass of ~ 3e31 kg and a radius of 7e11 m. A neutron star has a mass of maybe 1.5 M_sol = 3e30 kg and a radius of only 10 km, or 1e4 m. Therefore, the maximum acceleration due to gravity at the near side of the neutron star (relative to the supergiant) will be when the neutron star is right at the supergiant's edge. After that, when it enters the supergiant, it will feel a lower gravitational force, since less mass is between the neutron star and the supergiant core. This maximum acceleration is:
a_near = G m_SG/r_SG^2 ~ 0.004 m/s^2
The far side feels an acceleration of
a_far = G m_SG/(r_SG+r_NS)^2 ~ 0.004 m/s^2
Since the neutron star radius is so small, it's hard to see a difference unless I go out to a lot of significant figures. Doing a Taylor expansion, we can work out the difference in acceleration without calculating both answers to ridiculous precision. This difference is:
Delta a = 2 G m_SG r_NS/r_SG^3 ~ 6e-11 m/s^2.
Which is pretty piddly. For comparison, the gravitational acceleration at the neutron star surface is:
a_NS = 2e12 m/s^2.
Or about 200 billion g's. A gnat has a better chance of ripping my arm off than the supergiant has of pulling even one neutron off of this infalling dead star. Not surprising though: a neutron star is an atomic nucleus the size of Manhattan.

I was a bit surprised by how low the gravitational force of the supergiant is at the edge, but it is a huge object, and, per radius, not very massive. I.e., it's not a dense thing. Unlike the neutron star. Thus, the tidal force on the neutron star is negligible. Reversing this though, we see that the tidal effect on the supergiant material near to the neutron star is going to be ridiculously huge. The supergiant will get deformed as the neutron star approaches, even though the neutron star is getting pulled into the supergiant more than the supergiant is getting pulled into it. I imagine it would look like the supergiant going egg-shaped as the tiny neutron star falls in, followed by huge tendrils of plasma pulling up towards the neutron star. Pretty cool all around.
posted by physicsmatt at 8:39 PM on September 16, 2014 [11 favorites]

The neutron star has a ridiculously high self-gravity as compared that of a red giant. Or, really, just about anything.

So, give the neutron star a radius of about six miles. The red supergiant it fell into has a radius probably between 86,490,000 and 345,960,000 miles (200 to 800 times the radius of Sol), but with only about ten Solar masses. Just offhand, the neutron star is probably around 3 x 10²⁰ times as dense as the star it is plunging into. It may as well be an osmium bullet fired into the intergalactic space of the Local Void, for the difference in densities.
posted by adipocere at 8:39 PM on September 16, 2014 [1 favorite]

my first exposure to Vice was reading all about weird underground drugs and punk rock and trying to talk your girlfriend into anal sex. Now they're writing about binary stars. Life is weird.
posted by DGStieber at 8:57 PM on September 16, 2014 [1 favorite]

Better title for this article: "Astronomers Discover New Second-Most-Badass Type of Star (Black Holes Will Always Be Number One)"
posted by Anticipation Of A New Lover's Arrival, The at 6:02 AM on September 17, 2014

Back in a previous academic life, I was a theoretical astrophysicist who ran computational models of stars, and as such I read a couple of papers about TZOs. They were weird, obscure and very theoretical objects even to someone in the field. So it's pretty cool a) that one may have been found and b) that there are actual news articles about them now. (disclaimer: my PhD supervisor is involved in this research. It's a rather small field...)
posted by gnimmel at 6:49 AM on September 17, 2014

physicsmatt: The tidal effect on the supergiant material near to the neutron star is going to be ridiculously huge. The supergiant will get deformed as the neutron star approaches ... I imagine it would look like the supergiant going egg-shaped as the tiny neutron star falls in, followed by huge tendrils of plasma pulling up towards the neutron star.

There were a slew of press releases about this neutron star binary system (J1023+0038) where accretion from the companion star was caught in the act of turning on and off.

There's a bunch of nice associated images that show what you're imagining: Artist's impression 1, 2. (Obviously far earlier in the process, long before a TZO-type ending.)

There's even an animation on Youtube (but I have no idea about the provenance of that video - didn't even know it was there till just now).
posted by RedOrGreen at 7:25 AM on September 17, 2014