Credit: Illustration: NASA/CXC/M. Weiss
X-ray: NASA/CXC/Univ of Alberta/A. Bahramian et al.;
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. ...
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
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.
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.
Bruce
Just as zero is not equal to infinity, everything coming from nothing is illogical.
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.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
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.
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.
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.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
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.
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.)
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?
Bruce
Just as zero is not equal to infinity, everything coming from nothing is illogical.
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.