CXC: Star Discovered in Closest Known Orbit around Black Hole

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CXC: Star Discovered in Closest Known Orbit around Black Hole

Postby bystander » Mon Mar 13, 2017 5:49 pm

47 Tuc X9: Star Discovered in Closest Known Orbit around Likely Black Hole
NASA | SAO | MSFC | Chandra X-ray Observatory | 2017 Mar 13

47tuc[1].jpg

This graphic features an artist's impression of a star found in the closest orbit known around a black hole, as reported in our latest press release. This discovery was made using data from NASA's Chandra X-ray Observatory (shown in the inset where low, medium, and high-energy X-rays are colored red, green, and blue respectively), plus NASA's NuSTAR telescope and the Australia Telescope Compact Array.

Astronomers found this extraordinarily close stellar pairing in the globular cluster named 47 Tucanae, a dense collection of stars located on the outskirts of the Milky Way galaxy, about 14,800 light years from Earth.

This particular source, known as X9, has been of interest to scientists for many years. Until a couple of years ago, astronomers thought X9 contained a white dwarf pulling material from a companion star like the Sun. However, a team of scientists in 2015 used radio data to show that X9 likely consisted instead of a black hole pulling gas from a white dwarf companion. These researchers predicted that the white dwarf would take only about 25 minutes to orbit the black hole. ...

Star Discovered in Closest Known Orbit Around Black Hole
Michigan State University | 2017 Mar 13

Star in Closest Orbit Ever Seen Around Black Hole
International Center for Radio Astronomy Research | 2017 Mar 13

The Ultracompact Nature of the Black Hole Candidate X-ray Binary 47 Tuc X9 - Arash Bahramian et al

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Closest star-blachole binary pair

Postby shaileshs » Mon Mar 13, 2017 8:17 pm

http://www.space.com/36043-white-dwarf- ... video.html

I wonder why they say the scientists don't expect the star to fall into the black hole. What could be the reason ? Which stars are expected to fall into black hole and which ones aren't ? Why ? if they are expected to fall, how can we know how long it'll take (e.g. some time calculation based on gravitational pull of black-hole v/s mass of star etc) ? Thanks for all answers in advance.

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Re: Closest star-blachole binary pair

Postby BDanielMayfield » Mon Mar 13, 2017 10:14 pm

shaileshs wrote:I wonder why they say the scientists don't expect the star to fall into the black hole. What could be the reason ? Which stars are expected to fall into black hole and which ones aren't ? Why ?


For exactly the same reasons that the planets in our solar system don't fail into the sun. As long as the orbits are stable they can continue indefinitely. If some third body comes along it can rob energy from the system, causing the orbit to decay, which could rarely lead to mergers.

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Re: Closest star-blachole binary pair

Postby Chris Peterson » Mon Mar 13, 2017 11:04 pm

BDanielMayfield wrote:
shaileshs wrote:I wonder why they say the scientists don't expect the star to fall into the black hole. What could be the reason ? Which stars are expected to fall into black hole and which ones aren't ? Why ?


For exactly the same reasons that the planets in our solar system don't fail into the sun. As long as the orbits are stable they can continue indefinitely. If some third body comes along it can rob energy from the system, causing the orbit to decay, which could rarely lead to mergers.

The orbiting body also decays because it loses angular momentum to gravitational radiation. But that isn't significant until it's extremely close, and orbiting very fast (seconds or less). Another possibility is for the orbiting star to interact with an accretion disk around the black hole, which could also result in a loss of angular momentum and consequent spiral orbit. But at the scale of this system, it's pretty much as you describe it- a two body system, with the components orbiting around a common barycenter and very little to take energy out of the system.
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Re: Closest star-blachole binary pair

Postby rstevenson » Tue Mar 14, 2017 1:31 am

Chris Peterson wrote:...But at the scale of this system, it's pretty much as you describe it- a two body system, with the components orbiting around a common barycenter and very little to take energy out of the system.

The article linked to in the initial post of this thread says...
In order to make such a close pairing, one possibility is that the black hole smashed into a red giant star, and then gas from the outer regions of the star was ejected from the binary. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf.

That makes it sound as if the two are plenty close enough to lose angular momentum to gravitational radiation, so their orbits around their common center of mass will continue to shrink.

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Re: Closest star-blachole binary pair

Postby neufer » Tue Mar 14, 2017 11:06 am

rstevenson wrote:
Chris Peterson wrote:
...But at the scale of this system, it's pretty much as you describe it- a two body system, with the components orbiting around a common barycenter and very little to take energy out of the system.

The article linked to in the initial post of this thread says...
In order to make such a close pairing, one possibility is that the black hole smashed into a red giant star, and then gas from the outer regions of the star was ejected from the binary. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf.

That makes it sound as if the two are plenty close enough to lose angular momentum to gravitational radiation, so their orbits around their common center of mass will continue to shrink.
Click to play embedded YouTube video.

If the white dwarf wasn't constantly losing mass it would spiral in towards the BH in about 25,000 years.

The Hulse–Taylor neutron star binary has a much longer period of 465 minutes and it is given a inspiral gravitational radiation lifetime of only 300 million years.

I calculate that with a period of just 28.18 minutes the expected inspiral lifetime of this system is ~25,000 years [= 300,000,000*(28.18/465)(10/3)]

So the white dwarf must be losing mass at a faster rate than this. (Every time it loses half it's mass its inspiral lifetime doubles.)
Last edited by neufer on Tue Mar 14, 2017 7:53 pm, edited 2 times in total.
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Re: Closest star-blachole binary pair

Postby Chris Peterson » Tue Mar 14, 2017 2:19 pm

rstevenson wrote:
Chris Peterson wrote:...But at the scale of this system, it's pretty much as you describe it- a two body system, with the components orbiting around a common barycenter and very little to take energy out of the system.

The article linked to in the initial post of this thread says...
In order to make such a close pairing, one possibility is that the black hole smashed into a red giant star, and then gas from the outer regions of the star was ejected from the binary. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf.

That makes it sound as if the two are plenty close enough to lose angular momentum to gravitational radiation, so their orbits around their common center of mass will continue to shrink.

Yeah, I wasn't following closely enough to note just how close together these actually are.
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Re: Closest star-blachole binary pair

Postby neufer » Wed Mar 15, 2017 1:28 pm

neufer wrote:
Click to play embedded YouTube video.

If the white dwarf wasn't constantly losing mass it would spiral in towards the BH in about 25,000 years.

The Hulse–Taylor neutron star binary has a much longer period of 465 minutes and it is given a inspiral gravitational radiation lifetime of only 300 million years.

I calculate that with a period of just 28.18 minutes the expected inspiral lifetime of this system is ~25,000 years [= 300,000,000*(28.18/465)(10/3)]

So the white dwarf must be losing mass at a faster rate than this. (Every time it loses half it's mass its inspiral lifetime doubles.)

Well...I got the math right but screwed up on the physics. :oops:

Dimensionally the characteristic time: T = Energy / Power = [(1/Radius) / (Radius4/Period6)]
T = Period6/Radius5 = Period(18/3)/Period(10/3) = Period(8/3)
Ergo: Twd = (300,000,000 years)* Period(8/3) ~170,000 years [= 300,000,000*(28.18/465)(8/3)]

So the white dwarf must be losing mass at a rapid rate compared with a time scale of ~170,000 years.
(Every time it loses half it's mass its inspiral lifetime doubles.)
Art Neuendorffer

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Re: Closest star-blachole binary pair

Postby BDanielMayfield » Wed Mar 15, 2017 2:12 pm

neufer wrote:(Every time it loses half it's mass its inspiral lifetime doubles.)


Ergo: The incredible shrinking star. It will approach zero mass forever, but never get there. But, is this just from the pointless view from inside the ergosphere, or is it what an astronomer could see if the aging problem was solved?

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Re: Closest star-blachole binary pair

Postby neufer » Wed Mar 15, 2017 3:40 pm

BDanielMayfield wrote:
neufer wrote:
(Every time it loses half it's mass its inspiral lifetime doubles.)

Ergo: The incredible shrinking star. It will approach zero mass forever, but never get there. But, is this just from the pointless view from inside the ergosphere, or is it what an astronomer could see if the aging problem was solved?

The center of the white dwarf never gets anywhere near the event horizon or the ergosphere.

When the white dwarf loses half its mass the gravitational wave distortion also drops by half but the gravitational wave power drops by to one quarter. The white dwarf now has half its former energy but it's only losing that energy at a quarter the rate. Relativistic time distortion is irrelevant.
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