05.04 - 별 찌게 떠먹기(Sipping on Star Soup) [커세라 강의 페이지]
After examining Cygnus X-1's blue supergiant companion up close, we now shift our view to the black hole and companion star system as a whole. We know these companions can either be high mass blue stars like HD 226868 or low mass stars like our sun. But how exactly does a black hole eat from a companion star.
블랙홀과 동반성의 관계를 살펴보기로 하자. 크든 (무겁든) 작든(가볍든) 동반성은 블랙홀에 먹힐 처지다.
If the black hole is feeding from its companion star, then material from the star must be transferred to the black hole by some mechanism. The answer to this question is explained by the work of a French astronomer named Edouard Roche.
블랙홀이 동반성에서 물질을 흡수 한다면 작동원리가 있을 것이다. [공간에 떠도는 먼지를 중력으로 끌어들인다 처럼 간단치 않다. 중력 균형에 있던 동반성과 블랙홀의 쌍성계에서 한 별이 다른 별을 빨아먹는 데는 이유가 있어야 한다.] 이 질문에 대한 설명을 프랑스 천문학자 에듀와드 로슈의 연구에서 찾아볼 수 있다.
He developed a model for the transfer of material between two massive objects such as a star and a black hole known as the Roche lobe.
To understand the Roche lobe, let's consider two scenarios, a single star and a system of two stars. In either scenario the force of gravity will have some say on whether material will be drawn into the star or will not.
로슈엽 모형을 이해 하기 위해 두가지 시나리오를 상정해 보자. 단일별과 두별 시스템(쌍성계)이다. 두 시나리오 모두 중력이 물질을 끌어들일 것인지 아닌지에 따져볼 것이다.
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In the case of a single star, if we were to draw lines or contours of constant gravitational potential, we would need to create a series of circles originating from the star. These drawings are similar to the topographical maps of mountains.
단일 별의 중력 퍼텐셜 등위선을 그려보면 별을 중심으로 동심원이다.
How does this change when another star is nearby? When two stars are in a binary system, the gravity of the two will interact. These two stars would be in orbit around one another or rather around a common center of mass. However, we must also consider in addition to the gravitational force the force due to the relative motion of the stars, the centrifugal force.
인근에 다른 별이 위치하면 어떻게 될까? 쌍성계의 두별의 중력은 서로 작용한다. 두별은 질량 중심을 두고 서로 궤도를 돌고 있다. 두별 사이의 작용은 중력 뿐만 아니라 원심력도 있다.
Think about a child's roundabout in a play park. Once you kickoff and begin spinning, you can feel a force pushing you outwards. This is called the centrifugal force.
As a result of the rotation of the star system, we have gravity pulling inwards and centrifugal force pushing outwards. It is the combination of these two forces that are represented by the lines of constant potential in binary systems.
두별의 공전으로 인해 서로 당기는 중력과 밀어내는 원심력이 있다. 이 두힘의 조합된 쌍성계에서 포테설 등위선을 그려보자.
If we now build up lines of equal potential around two stars, we will initially see circles around each of these stars. However, as these rings get larger and closer together, their shape begins to change. They are slowly stretched in the direction of the opposing star. The circles begin to morph into teardrops. This stretch or distortion increases until they connect forming a figure of eight around the two objects. Each of these teardrops or lobes is called a Roche lobe. It is the Roche lobe for the star it contains. Any material that is inside the lobe is gravitationally bound to that star.
별에서 가까운 등위 포텐셜은 동심원이었다가 범위가 멀어지면서 상대별 가까이 가면서 모양이 달라진다. 물방울 모양을 그리는데 각 별을 둘러싼 두개의 물방울이 맞닫아 마침내 누운 8자 모양이 된다. 각각의 물방물 혹은 나뭇잎(lobe) 모양을 로슈 엽이라고 한다. 각 로슈 엽 안에 별이 자리하고 있다. 이 엽 안의 모든 물질은 그안에 자리한 별의 중력이 종속된다.
You can think about gravitational lobes like two lakes occupying adjacent valleys separated by mountains. The lakes' watersheds don't share any water unless they fill to mutual height which we typically call a watershed divide.
Similarly, material within a Roche lobe is bound unless there is a point where the potential is equal between the two stars. The point where these teardrops meet is known as the Lagrange point. The first Lagrange point is commonly labeled L1. If you're an astronaut situated in L1, you would feel an equal gravitational pull towards each of these stars but there are other points where we could feel the equal pull between the two stars.
If we continue to map the lines of equal potential, we find other Lagrange points. As you can see, there are four other points that surround a binary system.
Although we've used an example of two stars, you can draw similar lines of constant potential around any other pair of massive bodies including the Sun and the Earth but more importantly between a black hole and its stellar lunch. Lagrange points are special regions in space where the gravitational potential is relatively flat making it easy for spacecraft to hover there.
두 무거운 질량체 사이에 등위 포텐설이 일치하는 지점을 라그랑지 점 이라고 한다. 태양과 지구 사이 라그랑지 점에 태양 관측 우주망원경을 가져다 놓아 위치를 고정 시키는데 활용한다. 우리가 관심을 가져야할 부분은 이 지점이 태양과 지구 사이에도 존재할 뿐만 아니라 블랙홀과 그 동반성 사이에도 존재한다. [중력 균형이 있는데 어떻게 블랙홀은 동반성을 빨아먹을 수 있을까?]
Considering the Earth-Sun system, the first Lagrange point lies on their connecting line. This region known as L1 is used extensively as a parking spot for solar telescopes since they can hover at L1 using small amounts of fuel. As such this location has been used as a prime spot for astrophysical observations which we will discuss further in future modules.
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Black hole binaries are a type of system which contain two massive bodies and so these systems also have Roche lobes and Lagrange points, but how does this help us understand the transfer of matter?
The transfer of material from a star to a black hole is a gravitational effect. If material from the star moves outside its Roche lobe, gravity can pull it towards the black hole instead. But if material inside its Roche lobe is gravitationally bound to the star, then how can it move outside the boundary?
The easiest way for this to happen is if the star begins to fill its Roche lobe. As stars like our sun get older, they will swell up to become red giants.
At this time, the star can grow to fill its Roche lobe. At that point, material can spill over across the boundary at L1. The star stuff will end stop falling towards the black hole.
Stars can also fill left Roche lobe if the Roche lobe shrinks. How can this happen? The Roche lobes would get smaller if the bodies in the system move closer to each other or as astronomers call it the binary becomes more compact.
거성의 팽창하여 로슈 엽 한계를 넘으려면 로슈 엽의 크기가 아주 작아야 한다. 동반성이 블랙홀에 아주 가깝게 위치 해야 한다. 압축된 쌍성계 블랙홀이어야 한다.
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There are many ways that binary systems can become compact. One method involving gravitational radiation will be discussed in module 10. This image shows 16 black hole binaries that all live in our galaxy. At the top of the image, you can see our sun and the distance between it and the planet mercury. Mercury orbits the sun at just over a third the distance between our sun and the earth yet most of these systems are much smaller or much more compact.
쌍성계가 압축(아주 가깝게)되는 방법은 여러가지가 있다. 10주차 강의에서 다시 살펴보기로 한다. 천문학자들이 찾아낸 16개의 쌍성계 블랙홀의 규모를 보주는 그림이다. 축척을 비교하기 위해 태양과 수성의 크기를 함께 그려 놓았다.
So far we have looked at material being stripped away from the surface of the star as it overfills its Roche lobe. This material then crosses the first Lagrange point L1 and starts falling towards the black hole. This process is known as Roche Lobe overflow.
It can provide a fairly stable way of feeding a black hole for quite some time. But is it the only way to transfer mass from the star to the black hole? No. There is another option.
로슈엽 홍수 이외의 방법은 없을까?
Many stars have winds. Our sun does but its winds are considered puny by stellar standards.
Massive blue stars can blow away up to 100 million times more mass than our own sun does. Such strong winds can be captured by the gravity of the black hole and fall in towards the event horizon.
This type of accretion is known as Wind Fed. While Wind Fed mass transfer is only really an option for high mass stars, Roche lobe overflow can occur with any type of star as long as the companion star fills its Roche lobe.
Our good friend Cygnus X-1 is one such system with high mass companion that is both overflowing its Roche lobe and feeding the black hole with high velocity stellar winds.
우리가 주목하는 시그너스 X-1은 로슈로브 홍수와 항성풍 두가지 모두 작동하는 것으로 보인다.
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