Center of the Milky-way

In the past 20 years, astronomers have collected enough evidence through the observed motions of gas and stars to convince ourselves that something very massive lurks at the center of our galaxy.

Observe the resemblance to the face of a tiger. just below - left the position of the Sagittarius A Star

The first dynamical evidence came from the motions of the ionized gas streamers of the mini-spiral orbiting Sgr A*. Using the velocities of the gas estimated from the Doppler shift of spectral lines, astronomers estimated that a mass of six million solar masses must lie within 10 arc seconds of Sgr A*. This did not explicitly prove the existence of a black hole since that amount of matter could be accounted for by a high density of stars within such a large volume.

The two main groups devoted to tracking these stars were team of Andrea Ghez at UCLA, (using the 10-m Keck telescope on Mauna Kea, Hawaii) and Reinhard Genzel's team (using the 8-m VLT telescopes in Chile). Both groups take advantage of the high spatial resolution and sensitivity of these large telescopes to track the positions of the stars within the cluster using near-infrared images collected once or twice a year. Despite the large diameters of the Keck and VLT telescopes, air turbulence in the Earth's atmosphere blurs the images taken at the telescopes. On hot days, we can see the heat waves coming up off the ground, or if you look at a flame, you can see the heat influencing the air around it. This is what happens in our atmosphere.

In order to correct for it, astronomers are now using Adaptive Optics (AO) systems, which increases the sensitivity of observations. AO systems use double lasers from KECK I and KECK II to create an artificial laser guide star to better help the telescope focus on a particular location and account for the atmosphere's properties, taking advantage of some of the most advanced adaptive optics systems and techniques in the world. The optic electronics then apply corrections for the atmospheric turbulence before the data is recorded.

Thus very accurate stellar positions can be estimated in order to kept track of the motions of the stars in the compact central cluster which are zipping around Sgr A* at speeds up to 1400 km/s! Using Kepler's laws of motion, we use the orbital velocities and positions of the bright stars to estimate the mass that must be contained within their orbits. The resulting enclosed mass of 2.6 x 10^6 times the mass of the sun, combined with the minute size of Sgr A* constraint provided by the radio emission, suggests that the stars must be swiftly circling around a super massive black hole. In fact, the large number of observations of the stars orbiting the black hole has allowed us to provide the first even detection of the accelerations of the stars in the central cluster.

 Astronomers recently watched a gas cloud plunge near the hole, and now they’re watching several stars orbiting near it, in particular a star called S2 (sometimes SO-2). Until now, it was thought that SO-2 might be a double star. Two stars orbiting each other would have complicated the upcoming gravity test. But it was soon confirmed that S2 did not have a companion. S2 is young and about 15 times more massive than our sun. It is located 26,000 light-years away from earth in the center of the galaxy. S2 made its closest pass by the black hole during 2018. Astronomers used the star’s motion to try to confirm the mass of the black hole (now thought to be about 4.15 million solar masses).

And they’ll also use S2’s motion to try to confirm Einstein’s prediction that very strong gravitational fields should “stretch out” wavelengths of light, causing a gravitational red shift.

S2 orbits Sgr A* on an ellipse that takes about 15 years to complete. The diameter of its orbit is about 300 billion km, The orbit is an ellipse, the star drops down to a mere 18 billion km from the black hole, a positively terrifying close approach. That’s only four times farther from the black hole than Neptune is from our sun. The gravity of the black hole is so fierce it’ll accelerate the star to about 6,000 km per second — fast enough to cross the continental U.S. in less than a second. That close to a black hole, relativistic effects predicted by Einstein’s equations start to become important. For example, the light from the star will have to fight the gravity of the black hole to get to us, losing energy on its way out. In Newton's theory of gravity, orbits make perfect ellipses when they occur around single, large masses. However, in General Relativity, there is an additional precession effect due to the curvature of space - time, and this causes the orbit to shift over time, in a fashion that may be measurable with current equipment. This 3D visualization illustrates stellar motion in the galactic center at a particular instant in time.

It will be the first measurement of its kind. Gravity is the least well-tested of the forces of nature. Einstein’s theory has passed all other tests with flying colors so far, so if there are deviations measured, it would certainly raise lots of questions about the nature of gravity.

Observations of a sample of nearby galaxies reveal that such super massive black holes are not unique to the Milky Way. The formation of such a large black hole and how it affects the evolution of its host galaxy are not well understood, nor is the connection between the black hole in the Milky Way and those believed to exist in the cores of Active Galactic Nuclei (AGN's), which emit a huge amount of radiation from their nuclei at many different wavelengths. As the understanding of the Center of Milky way improves, our understanding of the movement of Solar System will also improve. This will give rise to a whole new field of research.

Small Brother of Black holes - The Magnetar

Our knowledge of the universe is always expanding, much like the universe itself. Discovery of the Magnetar, a powerful type of neutron star was first made in 1979. That year, astronomers suggested that certain blasts of gamma and X-ray radiation and radio pulses might be explained by stars with exceptionally powerful magnetic fields. Since then, astronomers have identified dozens of magnetars in and around the Milky Way. 

Stars go through a life cycle like everything else in the universe. What happens to a star at the end of its life depends on the mass of the star. For example, our sun is expected to grow into a red giant, then become a planetary nebula, then turn into a white dwarf star. More massive stars can explode into supergiants, erupt into supernovae, and then become either a neutron star or a black hole.

Magnetars are the remnants of those massive stars which have exploded in a supernova and collapsed into a neutron star. While astronomers don't yet know what causes a supernova to result in a magnetar instead of a "normal" neutron star or pulsar, some hypothesize that it has to do with the original star's rotational speed. Magnetars are neutron stars with fields of approximately 1013 to 1015 Gauss (a measure of magnetic density). This is a scale of magnetic power that's hard to conceive, but let's just say that magnetars are considered to be the most powerful magnetic objects in the known universe.

Scientists have confirmed the presence of 23 known magnetars, and another six are waiting additional data to confirm if they meet the criteria to be considered magnetars. Many of these are located in the Milky Way, but don't worry: None are close to Earth!

Some of the magnetars near Earth include AXP 1E 1048-59, which is located about 9,000 light-years away in the constellation Carina; SGR 1900+14, 20,000 light-years away in Aquilla; SGR 1806−20, 50,000 light-years away in Sagittarius; and SGR 0525−66, 165,000 light-years away in the Large Magellanic Cloud (just outside the Milky Way). These distances are obviously far beyond anywhere we've explored in our galaxy – or even sent probes like Voyager 1 or 2 to visit.

Magnetars vs. Black Holes

Black holes definitely get a lot of headlines – and they're certainly not the kind of thing we'd want close to Earth. But are they more powerful than magnetars, which are the most powerful magnets in the universe? Phil Plait, an astonomer who shares his insights under the moniker Bad Astronomer, says in an email that it depends on what force you're measuring.

"The gravity from the black hole will always be stronger, because the lowest mass black hole is always more massive than the most massive neutron star," Plait says. "But the magnetism of the magnetar will be stronger, in general."

Luckily, we'll never have to worry about encountering a black hole or a magnetar close to Earth, but both could theoretically impact us here on Earth. "If a stellar mass black hole eats something it could blast out radiation, but even then I doubt it would be as strongly felt from halfway across the galaxy as the 2004 magnetar event," says Plait, referring to the massive gamma and X-ray blast that passed over Earth that year and caused disruptions to satellite technology, among other issues.

Do We Need to Fear Magnetars?

If you ask an astronomer, many will say that magnetars are among the scariest objects in the galaxy. Certainly you don't want to be near one – but the massive blasts of energy they produce can impact us here on Earth despite their great distance away. "I am worried about magnetars, given what happened in 2004," says Plait. "[SGR 1806-20] is exceptionally powerful. I don't think any that strong are closer [to Earth], but the impact on Earth gets stronger with the inverse of the distance squared. If one were one-fifth that distance the impact would be 25 times stronger."

As astronomer Paul Sutter points out in his 2015 article in Space.com titled "Why Magnetars Should Freak You Out," not only would a strong magnetar pulse affect our electronics and technology, but one with enough strength would affect our physiology, including the bioelectricity in our bodies — and between the atoms that make up everything we know. Let's just say we should all be glad that the nearest known magnetar is 9,000 light-years away.

Stupendously Large Black Holes (SLABS)

Studies in 2020 suggested the possible existence of 'stupendously large black holes' or SLABS, even larger than the supermassive black holes already observed in the centers of galaxies. The research, led by Queen Mary Emeritus Professor Bernard Carr in the School of Physics and Astronomy, together with F. Kühnel (Münich) and L. Visinelli (Frascati), investigated how these SLABs could form and potential limits to their size.

Whilst there is evidence of the existence of supermassive black holes (SMBHs) in galactic nuclei—with masses from a million to ten billion times that of the Sun—previous studies have suggested an upper limit to their size due to our current view on how such black holes form and grow. The existence of SLABS even larger than this could provide researchers with a powerful tool for cosmological tests and improve our understanding of the early Universe.

Challenging Existing Ideas

It has widely been thought that SMBHs form within a host galaxy and grow to their large sizes by swallowing stars and gas from their surroundings or merging with other black holes. In this case, there is an upper limit, somewhat above ten billion solar masses, on their mass. In this study, the researchers propose the possibility how SMBHs could evade this limit. They suggest that such SLABs could be 'primordial," forming in the early Universe, and well before galaxies come into existance. As 'primordial' black holes don't form from a collapsing star, they could have a wide range of masses, including very small and stupendously large ones.

Professor Bernard Carr said: "We already know that black holes exist over a vast range of masses, with a SMBH of four million solar masses residing at the center of our own galaxy. Whilst there isn't currently evidence for the existence of SLABs, it's conceivable that they could exist and they might also reside outside galaxies in intergalactic space, with interesting observational consequences. However, surprisingly, the idea of SLABs has largely been neglected until now."

Understanding Dark Matter

Dark matter is thought to make up around 25 percent of the ordinary mass of the Universe and Dark Energy comprising about 70% with balance of 5% comprising of the visible universe. Whilst we can't see it, researchers think dark matter exists because of its gravitational effects on visible matter, such as stars and galaxies. However, we still don't know what the dark matter is.

"SLABs themselves could not provide the dark matter," said Professor Carr, "but if they exist at all, it would have important implications for the early Universe and would make it plausible that lighter primordial black holes might do so."