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Supermassive Black Hole Causing a Stir in the Milky Way

May 18, 2012 2Comments

Have you ever wondered what goes on in outer space, especially around black holes? I know as a kid, viewing black holes as giant vacuum-like structures, I wondered about the kind of things that might be getting sucked into them while drifting through space. Well, a few weeks ago, I heard a talk by Dr. David Ballantyne about what is actually happening in space around supermassive black holes (SMBHs). (cue Twilight soundtrack)

Black holes are categorized based on their number of solar masses, where one solar mass is equal to 1.989×1030 kg. In order for a black hole to be considered supermassive, it must be greater than 105 solar masses. Goodness gracious! That’s the equivalent of several million or, in some cases, several BILLION suns. A BILLION suns?!? Yes, folks. A billion. And while the formation of black holes isn’t completely understood, it’s possible for black holes to add to one another and form new, supermassive black holes (known as merging).

One location of particular interest is the center of the Milky Way galaxy at Sagittarius A* (pronounced “Sagittarius A star”) where a SMBH potentially resides. Abbreviated Sgr A*, this radio source is about 2.5×104 light years away from our solar system. It can be hard to see due to the murky clouding around it, but with advancements in adaptive optics and the use of telescope arrays, astrophysicists have been able to track young stars orbiting around the area— a key indicator that a black hole is present. In the animation below, you can see various stars orbiting around Sgr A* (the stationary star shape). Star S0-2 is particularly cool to watch as it can be seen making a complete orbit:

 

 

Another interesting thing scientists have found is a dense cloud of gas which is moving directly toward this same SMBH. As the gas nears the black hole, part of its structure gets sucked inside. As you can see in the video below (at around 3:22), the cloud will have its closest encounter with the black hole in 2013. That’s next year! As the gas gets sucked into the SMBH, the black hole will gain energy and begin to glow brighter.

 

 

Black holes are typically found at the center of galaxies organized in structures referred to as active galactic nuclei (AGNs), which are comprised of the black hole and any surrounding material, in addition to any gas or radiation present. As the material near the black hole swirls around, it creates what is known as the accretion disk. Like our sun, whose corona emits x-rays, black holes can also emit various forms of radiation, including x-rays. What?! Yessir! In a process called magnetic reconnection, magnetic field lines interact and reconnect with one another, causing (x-ray radiating) plasma to be emitted. These x-rays are then dispersed into space, some of which hit the accretion disc and create a ‘reflection spectrum’ which scientists can study.

 

 

Now, while supermassive black holes weigh millions (sometimes billions) of suns and have the ability to tear apart nearby objects in space, you don’t have to worry about being sucked into one any time soon. Unless you’re this guy, in which case you may have some issues…

 

 

MUSE’s “Supermassive Black Hole”:

 

 

Sources:

UCLA Galactic Center Group (http://www.astro.ucla.edu/~ghezgroup/gc/)

Special thanks to Dr. Ballantyne

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Katelyn Sturdivant
About Author

Katelyn is in her 4th year at Georgia Tech. In her spare time, she likes to hang out with her family, volunteer at Kennestone Hospital, and be crafty. She is a pescatarian and is currently trying to learn the F chord on her acoustic guitar, Gretchen. Katelyn hopes to attend both medical school and graduate school (for a Master's in Public Health).

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.