09.06 - 은둔자 블랙홀(The Hermits of the Black Hole Family) [커세라 강의 페이지]
Until now, we have only discussed black holes that are either in a binary star system, or located at the center of a galaxy. Surely, those are the only ways to find black holes. Indeed, scientists think that there are isolated black holes lurking within our galaxy. So, how do we detect them?
지금까지 쌍성계의 블랙홀, 은하중심의 블랙홀에 대해 이야기 했다. 이런 블랙홀들은 [직접적이진 않지만] 관측 할 수 있었다. 과학자들은 우리 은하 여기저기에 잠복한 독립 블랙홀이 있다는데 동의한다. 그럼 이 블랙홀을 관측할 방법은 무엇인가?
We need to remember the presence of gas between stars called the interstellar medium. If an isolated black hole travels through a gas cloud, we would expect to see some of the gas to accrete onto the black hole. The accreting gas should emit x-rays, which could potentially be detected.
성간물질이 블랙홀로 강하게 빨려들어가면 X선이 방출될 것이다.
Although astronomers detect lots of x-rays emitted from gas clouds, conclusive evidence for isolated black holes using this method hasn't been detected.
비록 천문학자들은 가스 구름에서 X선이 방출되는 것을 관측하긴 했지만 이것이 블랙홀의 증거라고 결론지을 만한 것을 찾아내진 못했다.
If there isn't light being generated by an isolated black hole, is it still possible to detect them? Of course. Black holes change the gravitational field in their local environment. So, light passing by is influenced by the gravitational field. Since black holes can strongly warp space-time, they cause light to travel on curved paths. We see the light from behind the black hole as being warped by gravity. This is called gravitational lensing.
독립 블랙홀이 빛을 방출하지 않는다면 블랙홀을 발견할 다른 방법은 없을까? 물론 있다. 무거운 블랙홀이 주변의 시공간을 왜곡하여 뒷편의 별빛을 휘게 만든다. 바로 중력 렌즈 라는 것이다.
Here's a computer-generated starscape showing the constellation of Orion along with the bright stars Procyon and Sirius. Brighter stars are represented by larger circles, and dimmer stars by smaller circles. Light can't escape if it enters the black hole's event horizon. So, if a black hole were to get in the way of our view of Orion, we might see something like this computer generated image, with a dark circular region where we see no stars.
The black circle corresponds to the event horizon of the black hole. The black hole is blocking our view in the same way a cat blocks your view when it decides it's time for attention. But if we look carefully at the area around the dark region, it looks as though many more stars have appeared around its outer edge. The strange appearance of these additional stars is an optical illusion. What we're actually seeing is multiple images of the stars that reside in the background. For instance, you should be able to see two images of the group of three belt stars on either side of the black hole. Similarly, you can see two images of the very bright star called Sirius. What we would see is more complicated than just a black circle in the sky. Instead, light from the faraway stars curves around the black hole, and arrives in our eyes as though the star is located at many locations. The black hole's gravity distorts the image of the background stars, giving away its presence. In reality, there is no black hole close enough to give us a view like this distorted picture of Orion, but we can use the concept of gravitational lensing to identify some isolated black holes.
If this lensing effect is difficult to wrap your head around, you probably feel like a photon that's been bent in the space-time around a black hole. Let's have a look at a real-world example of image distortion by your dining room glasses. Stemmed glassware are simple vessels that can be filled with a liquid. Since an empty glass has curved edges, it bends and warps light just like a lens. When the glass passes in front of a picture, like this star map, a distorted view of the background becomes visible. Since the curvature of the glass allows multiple images of the same object, we see multiple images of some stars. Depending on where the stars are, they can also be distorted into rings.
When gravity is weak, light travels on paths that we consider straight. Since gravity can distort space-time, photons are forced to travel along geodesics, which are curved paths that bend around the black hole. To us, the resulting images have multiple views of the same object along with great arcs of refracted lights similar to what we saw in the glasses. For this reason, when the gravity of a massive object curves the path of light, we call the massive object a gravitational lens. A gravitational lens can be a star like our sun, a galaxy, or a black hole. The more massive, the better.
When the faraway object, the nearby mass, and the earth have perfect alignment, the image that we see is called an Einstein ring. This diagram demonstrates the perfect alignment between the Earth and nearby galaxy at a faraway star. Light from the star can be emitted on paths that go around the galaxy and reach the earth in many different ways. The light can go over or under or beside the galaxy. The result is that the light in our telescope from the distant star looks like it comes from a ring in the sky that surrounds the nearby galaxy. Here is an example of an Einstein ring captured in a photo taken by the Hubble Space Telescope. The fuzzy orange blob in the center of the picture is a nearby galaxy. The blue circular yellow is a distant galaxy lying behind the orange galaxy. The mass of the nearby orange galaxy warps space time, and the light from the faraway blue galaxy appears like a ring due to the gravitational lensing effect. The ring is not a perfect circle due to the fact that the orange galaxy's mass is not located at one point.
If the source of light and the lens' mass do not line up perfectly, we see multiple copies of the same faraway galaxy. In this picture, we see four lights that are all images of the same faraway quasar that is behind the nearby galaxy. The nearby galaxy is the fuzzy light in the center of the four quasars. This is called an Einstein Cross since it resembles a cross.
When astronomers view images of gravitational lensing, they can compute the mass contained in the nearby galaxy. In many cases, the mass calculated from gravitational lensing is larger than the mass inferred from looking at the bright stars. This is one method used to show the existence of dark matter in galaxies. Although black holes could be a type of dark matter, most dark matter is not made of black holes.
The size of an Einstein ring is related to the mass of the nearby object. For the images that we've shown, the nearby mass is a galaxy and galaxies have gigantic masses larger than 100 billion suns. The large mass gives a large deflection and a bigger looking Einstein ring. If the lens mass is small, where small means similar to the sun's mass and the distance is far from us, then the size of the ring will be too small for a telescope to resolve. Instead of seeing a ring, the light from the faraway star will appear brighter. This situation is called gravitational microlensing. The mass causing the lensing could be a dim star like a brown dwarf or a black hole.
Astronomers have been monitoring many stars in a nearby galaxy to look for the microlensing brightening effect due to an isolated black hole in our galaxy. If the black hole is traveling between the Earth and the faraway stars, then they will appear brighter while the black hole is in front of them. This gives us a view of a star that appears brighter for a short time period.
Microlensing by black holes is very rare, but it is seen occasionally. This image shows a gravitational lensing event that occurred in 1996. The top panel shows a dim star on April 28th and the bottom panel shows the same star on November 15th. The image on November 15th is brighter and a further analysis show that the star appears brighter because a black hole with a mass around ten times larger than the sun passed by. Only a handful of black holes have been found this way since this is a really rare event.
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[인터뷰] [커세라 페이지]
What does microlensing have in common with exoplanet searches?
Interview with Dr. Kelsey Hoffman, Researcher at the SETI Institute
So there is two different main aspects of it. One is looking for objects, whether they be planets or compact objects. The other part is understanding the structure inside these objects. So first of all, the looking for them is through the transit method, where you have your source, like your sun-like star and a planet that orbits in between you and the sun, or the star. When a planet does this though, you'll see the light from the source decrease. Now when it comes to compact objects, and you may be replaced like an Earth-like planet with a white dwarf which has the same radius. When that is going in front because of the mass of that white dwarf, it actually acts as a lens and magnifies what you see. Then in this case, you get something instead of the light going down, it increases in what we call microlensing. So in the gravitational lensing, the mass of the object allows the light will now bend around the object instead of being blocked by it. In these cases you get an amplification because the light bending will create different images of this and so then it increases the light that you see.
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