Most people when asked which is the brightest star in the sky, would point to Sirius which is indeed the most luminous star in the night sky. However, the brightest star in the sky is the Sun, which we tend to forget while answering the question. Similarly, when it comes to supermassive black holes, we usually associate them with active galaxies many millions of light years away. But an object which is very likely to be a supermassive black hole, is present right at the centre of our own galaxy, the Milky Way or Akashganga. The presence of this object was established by meticulous observations made over a period of ten years beginning around 1992. Important results were published in 2002-03, but the observations have continued right to the present, reinforcing the early results and producing many new ones.
The work leading to the finding of such a supermassive object was done independently by Professor Reinhard Genzel and his group in Germany, and Professor Andrea Ghez and her group in the USA. Genzel and Ghez were awarded half the Nobel Prize for Physics in 2020 for their efforts. The other half of the prize was awarded to Professor Roger Penrose, as mentioned in Part 1 of this article.
The Milky Way is a spiral galaxy with more than a hundred billion (1011) stars. A large fraction of these stars is distributed in a relatively thin disk, rotating around the centre of the galaxy. The diameter of the disk is about a hundred thousand light years. The second prominent structure of the galaxy is known as the bulge. It is nearly spherical in shape with diameter smaller than the disk. The stars in the bulge move around the centre, but not in a plane as in the case of the disk. In addition to the bulge and the disk, there is a much larger, nearly spherical structure, known as the Galactic halo. The disk has the large spiral arms which give spiral galaxies their name. Here, we will only focus on the stars which are very close to the centre. An image of the Milky Way galaxy is shown in Figure 1.
Fig. 1: This is a panoramic view of our galaxy in near-infrared light obtained by mosaicking images from the 2MASS survey (Image Courtesy: IPAC). The disk and the bulge are seen, though not the spiral arms, because we have only an edge-on view of our own galaxy. The halo is not bright enough to be seen in the image.
It would be quite natural for a massive object to be present at the centre of the galaxy, as we mentioned in Part 1. Such an object would influence the motion of the stars in the galaxy through the gravitational force it exerts on them. The total gravitational force acting on a given star, say the Sun, depends on the mass of the compact object, as well as the mass of the stars inside the orbit of the Sun. The closer a star is to the centre of the galaxy, the greater would be the influence of the compact object, as much of the mass of the stars would be left behind. If the motion of the stars, very close to the centre of the galaxy, could be observed, it would be possible to determine the shape of the orbit of the stars. Then using Newton’s law of gravitation, the mass of the object can be determined.
The devil is in the details.
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THE GALACTIC CENTRE
On a clear dark night, the band of the Milky Way moving across the night sky is easy to spot. The bulge of the galaxy, which includes the central regions, is located in the constellation of Sagittarius and can be seen as a large, faint nebulosity with dark patches in it. The central region is a complex environment with high density of stars in a cluster around the centre. Stars which are being formed, exploding stars, gas and dust. A source emitting radio waves has, for long, been known in the central region. In 1974 Bruce Balick and Robert Brown discovered a very compact component of radio source, which was later named Sagittarius A* (Sgr A*). Observations have shown that its size is smaller than the distance between the Sun and the Earth, which is about 150 million kilometers, a size which is very tiny compared to the scale of the galaxy. Sgr A* shows no motion, which suggests it is massive, and located at the centre of the galaxy.
In fact, the mass of the central object can be measured by determining the orbits of the stars in the galaxy. Observing individual stars in this region is very difficult for two reasons: First, the centre is about 25,000 light years away from the Sun, which makes the stars appear very faint. This dimming is made worse by the presence of dust in the centre, which obscures the stars making them look much fainter than would be warranted by their distance from us. Second, the stars in the centre are crowded together, so they seemingly blend with each other, and are difficult to be distinguished as individual objects. To get over these difficulties, it is necessary to use the largest telescopes and observe in the near-infrared region of the spectrum, where the obscurations due to the dust is low. It is also necessary to use specialised techniques like adaptive optics and speckle interferometry to increase the resolution of the observations, so that individual stars can be distinguished. All these high technologies make it possible for the motion of the individual stars close to the centre of the galaxy to be followed over a period of time, establishing the shape of their orbit.
Saggitarius A*
The center of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*), located in the middle, is revealed in this image. Astronomers have used NASA’s Chandra X-ray Observatory to take a major step in understanding why material around Sgr A* is extraordinarily faint in X-rays. The large image contains X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow.
The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards. This hot gas originates from winds produced by a disk-shaped distribution of young massive stars observed in infrared observations.
These new findings are the result of one of the biggest observing campaigns ever performed by Chandra. During 2012, Chandra collected about five weeks worth of observations to capture unprecedented X-ray images and energy signatures of multi-million degree gas swirling around Sgr A*, a black hole with about 4 million times the mass of the Sun. At just 26,000 light years from Earth, Sgr A* is one of very few black holes in the universe where we can actually witness the flow of matter nearby.
Image Courtesy: Chandra X-Ray Observatory
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THE ORBIT OF STAR S2
Determining the path of a star requires its position to be known at different times over a long period of time. Imagine determining the path of a planet like Mars or Jupiter in the sky. The position of the planet at given times can be determined in comparison with the position of the neighbouring stars in the sky. The stars can be taken to be stationery, since they are much further away from us than the planets, and their motion is negligible compared to the motion of the planets. In practice, various coordinate systems have, over centuries, been developed relative to which the motion of planets, and even the tiny motion of stars can be determined with extreme accuracy.
Such measurements are much more difficult in the galactic centre, as the stars there, are in incessant motion and it is extremely hard to determine the position of any one star over a period, which stretches to years. The reference stars used for the purpose are several which are known to be radio emitters and whose position at a given time can be determined very accurately using an array of radio telescopes spread over the Earth. With this method, the position of a star can be determined to an accuracy of about ten millionth of a degree.
Reinhard Genzel (Left) and Andrea Ghez (Right) were awarded half the Nobel Prize for Physics in 2020 for their work in identification of supermassive black hole (Picture: TEd)
Reinhard Genzel’s group used the Very Large Telescopes (VLT, there are four of these, each with a primary mirror of 8.2 metres in diametre) and the New Technology Telescope (3.58 metres primary) of the European Southern Observatory (ESO) located in Chile, for their observations. Andrea Ghez and her group used the Keck Telescopes (there are two of these, each with primary of 10 meters diametre) located in Hawaii. Both the groups followed the motion of several stars in the cluster, with the best results obtained for a star designated as S2, which, during its motion, very closely approached the central object. We will describe the results obtained for this star over the years. beginning with results based on about a decade of observations between 1992-2002 and published by the two groups in 2002-03.
Shown in figure 2 are the results of the motion of star S2 obtained by Genzel’s group. The red image on the left shows a number of stars from around Sgr A*. The position of S2 at various times from 1992 to 2002 are on the right of the figure, along with the position of Sgr A*. The short bars on each point indicate the extent of the possible errors in the position. The path of the star over the years is indicated by the ellipse, which is obtained as the best representation of the data. It is found that Sgr A* is located at one focus of the rather elongated ellipse (every ellipse has two foci). From Newton’s theory of gravitation, it follows that a large mass located at the focus, coincident with Sgr A*, should be exerting a gravitational force on star S2, which makes it follow the elliptical orbit. Calculations showed that the mass of the object is close to 3.3 million times the mass of the Sun. The time taken by S2 to complete one orbit, known as the period of orbit, is about 15.6 years and the closest distance S2 approaches Sgr A* is about 0.73 light days, that is about 19 billion kilometres. The estimate of the mass has been found to increase as corroborated by additional data. It is now 4.3 million times the mass of the Sun.
Fig. 2: Motion of S2 on its elliptical orbit around Sgr A* between 1992 to 2002 (Image Courtesy ESO)
The elliptical orbit of S2 should remind us of the momentous result obtained about 400 years ago by Johannes Kepler on the orbit of Mars. Kepler set out to determine the path of Mars in the sky, using data collected by astronomer Tycho Brahe, who was Kepler’s supervisor. With heroic effort Kepler established that the orbit of Mars was elliptical in shape, and that the Sun was at one focus of the ellipse. Kepler could not determine the mass of the Sun, as Newton’s discovery of the universal force of gravity happened about a century after Kepler’s work. But Kepler discovered his three famous laws of planetary motion, which were later explained using Newton’s theory.
THE IMPERFECT ELLIPSE
The two groups of astronomers continued to monitor S2 beyond 2003. The later observations provided data points which covered the whole ellipse, as they were taken over a span of time longer than one orbital period. A complete orbit is shown in Figure 3. An ellipse is again seen to provide a very good representation of the data points, but a close look shows that the ellipse is not quite perfect. At the top of the figure, it is seen that the ellipse does not quite close on itself, there is a small gap. The gap indicates that there is a slight rotation in the ellipse from orbit to orbit.
What is this due to?
A hint is provided by the position of Sgr A* at the focus, towards the bottom of the ellipse. We know that Sgr A* is a very compact source, so it should appear as a point in the diagram. But the source appears to be slightly extended. This is interpreted as a slight motion of the gravitating mass. Such a motion would produce a deviation from a perfect elliptical orbit, as is observed, and seems to provide the right explanation. But later observations showed that the deviation is due to quite a different effect.
In Einstein’s theory of gravitation, which we introduced in Part 1, the orbit of a body around another gravitating body is nearly elliptical, but not completely. Over successive orbits, the effect of the theory is to produce a slight rotation of the ellipse. This is known as the precession of the perihelion, because the shift in the ellipse means that the point at which the two bodies are closest shifts from orbit to orbit. A result of the shifting ellipse would be the kind of gap which is observed.
Could this be the reason for the imperfect ellipse traced by S2?
Recently, the accuracy of the measurements of the position of S2 over time has improved, which is more than tenfold because of the availability of a new instrument called GRAVITY, that combines the optical beams from the four VLTs. Figure 4 shows the results obtained by the GRAVITY team, combined with earlier observations. The grey curve, which is a near-perfect ellipse, except for the small deviation near the top as before, has been obtained by calculations which use Einstein’s theory. The observed points are very well aligned with the curve, which shows how precise the observations are.
The black cross towards the bottom of the ellipse indicates the position of the massive compact object. The red crosses indicate infra-red flashes observed around Sgr A*. In this new data set, there is no evidence that the object is moving. It is stationary. So, the deviation of the ellipse for a perfect form is wholly explained on the basis of the effect of Einstein’s theory. This is a remarkable result, and is the first observed precession in the context of a supermassive object. Such precession has previously been observed in the orbit of Mercury around the Sun and in binary pulsar systems.
Animation of the precession (Video courtesy: ESO)
There is another prediction of Einstein’s theory of gravity which has been observed in S2. Light, which is emitted by S2, in reaching us, has to emerge from the strong gravitational field of the compact object. In doing so, light gets redshifted i.e., its wavelength increases by an amount which can be calculated from Einstein’s theory. The spectrum of the star S2 was observed on 19 May 2018, when S2 was at its closest point to the compact object. The observed redshift in the features in the spectrum was exactly as predicted by the theory. The passage of S2 is shown in figure 5. The speed with which S2 moves around the compact object increases as it moves closer to the object. At periastron, S2 moves with a speed in excess of 25 million kilometers/hour, which is more than about 2.3 percent of the speed of light. At such speeds the effects on the spectrum related to Einstein’s special theory of relativity are large, and have to be considered while measuring the gravitational redshift. The gravitational redshift result was obtained independently by the ESO GRAVITY experiment group and Andrea Ghez’s group.
Fig. 5: Passage of S2 (Figure Courtesy ESO).
Genzel and Ghez set out to measure the mass of the compact object at the centre of the Milky Way galaxy. Their remarkable nearly three decades long work has been highly successful in meeting its original goal, and other marvelous results have emerged from it too. Many more results will follow in the future. It is not surprising that they were awarded the Noble Prize in Physics for their efforts.
HISTORICAL NOTE
The notion of the presence of a supermassive black hole at the centre of galaxies became popular after the discovery of quasars, active galactic nuclei, super luminal motion etc. pointed to the existence of such objects at the centres of galaxies. In the mid-1960s, before many of these observations were made, Fred Hoyle and Jayant Narlikar had considered the existence of objects, with mass of about a billion times of the mass of the Sun, around which matter would condense to form a galaxy. They predicted from their model that there would be highly concentrated points of light in the centres of elliptical galaxies. This is probably the first mention of a supermassive object at the centre of galaxies.
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IS THE COMPACT OBJECT A SUPERMASSIVE BLACK HOLE?
What is the nature of the compact object?
We have seen that the closest distance of approach of star S2 to the gravitating object is about 19 billion kilometres, so the size of the object should be much smaller. If the compact mass is a black hole, then it would have zero radius. But if it is not, it would have a non-zero small radius. What could such an object be? The object is not emitting much light, so it must be made up of a large number of small dark objects, like stars of very low mass, or neutron stars, or even stellar mass black holes which are the remnants of stars, as mentioned in Part 1. But calculations show that any such aggregate would have very high density at the centre, and would collapse under its own gravity, or simply dissipate, in less than a million years, which is far, far smaller than the known age of the galaxy, which is more than 15 billion years. More exotic possibilities like a dark mass made up of exotic particles like neutrinos, gravitinos, axinos, etc. or a ball of bosons are also possible. But it is presently not possible to produce robust models of such objects consistent with known theory and observations. It appears, therefore, that most of the mass of the compact object at the centre of our galaxy consists of a supermassive black hole, with the remaining mass being due to a star cluster around it. The GRAVITY instrument and other future instruments will enable us to penetrate deeper towards the object, possibly revealing effects which could be unambiguously attributed to a supermassive black hole.
AJIT KEMBHAVI is an Indian astrophysicist with research interests in galaxies, high energy astrophysics, Big Data and applications of Artificial Intelligence to astronomy. He can be reached at akk@iucaa.in
Opinions expressed in this article are of the author’s and do not represent the policy of The Edition. The writers are solely responsible for any claim arising out of the contents of their articles.
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