Black holes, Neutron Stars, and White Dwarfs

The universe is an amazingly entity that scientists have studied for hundreds of years.  Much about our universe has been thoroughly explained but yet there is still so much more that has to be explained.  Stars are arguably the most researched objects in our entire Universe.  Since the dawn of time stars have been visible to the people on earth. Stars are so very interesting to people because they are extremely complex. From the beginning to the end of a stars life cycle there is so much to observe and so much to document.  Stars all begin life the same way but the end of the life cycle of a star is the interesting part. Depending on many different variables a star can end up as a white dwarf, neutron star, or a black hole.  The biggest deciding factor on how a star will end its life, according to many astronomers, is its mass. The Sun and any star with a mass that is around the same as our Sun’s mass will end its life as a white dwarf.  If a star after its final stages of life reaches a mass of over 1.4 solar masses it will become a neutron star. Black holes are produced at the end of a very massive star’s life cycle.  

Above is just a general image of a white dwarf and below is a white dwarf in a binary system that is gaining mass (accretion) from a red giant.

White dwarfs are the most common final stage in the life cycle of most stars in the universe.  There are many more stars in the universe with lower masses (similar to that of our sun’s) then there are massive stars so there are a lot more white dwarfs than neutron stars or black holes (3). A white dwarf is a very dense solar remnant that is supported by the balance between electron degeneracy pressure and the star’s gravitational self-attraction (3).  A white dwarf’s mass is about that of the sun’s mass but what makes them so interesting is that their size (diameter) is about that of the earth. 

White dwarfs are a very interesting part of stellar evolution.  A white dwarf, when it is first produced, burns at the bottom left of the Hertzsprung-Russel diagram because of this astronomers know that white dwarfs are very hot but not very luminous when they are first produced. As white dwarfs age they begin to cool and if no other forces act on them they eventually become virtually non-existent and they produce no radiation.

White dwarfs are sometimes found in binary systems.  In a binary system two objects (in this case stars) are so close together that they gravitational interact with each other and the gravitational interactions cause the objects to rotate around a common center of mass.  When a white dwarf is found in a binary system typically the second star is a red giant. After sometime the red giant can begin to lose mass, when this occurs the white dwarf accretes matter from the red giant.  If the white dwarf gains enough mass to surpass the Chandrasekhar limit, which is 1.4 solar masses, the electron degeneracy pressure can no longer balance the star’s gravitational self-attraction (3). Thus resulting in a very bright type 1a Supernova (3). 

Above is a Neutron star, I pulses, that's why the image looks the way it does.

            Neutron stars are remnants that form from stars that had initial masses of 4-8 solar masses. These stars go through supernova explosions near the end of there lives and they blow off matter and then form neutron stars of anywhere from 1.4 solar masses to 3.2 solar masses (1).  Neutron stars are supported by neutron degeneracy pressure.  Neutron stars rotate extremely fast when they are first formed, and they later slow down. They rotate fast initially because when the core of a massive star is compressed and it collapses into a neutron star its angular momentum is conserved and the size (diameter) of the star has been reduced to about the size of Ann Arbor. Astronomers have calculated neutron star periods of between 1.4 ms and 30 seconds.  Many astronomers refer to them as pulsars because when observed they appear to pulse because of their very fast periods. A neutron star’s gravitational field is so immense that the escape velocity is approximately 1/3 the speed of light or 10^5 km/s (5).

            Neutron stars can also accrete matter from other objects in space. When neutron stars accrete matter they get smaller as the neutron degeneracy pressure grows more intense.  Neutron stars do have a similar limit to that of white dwarfs but when they reach their limit they behave quite differently (5).  When neutron stars gain matter and neutron degeneracy pressure is over thrown by gravity, a neutron star can form a black hole (5).  

Theoretically this is what a black hole wold look like but of course we can't see a black hole but we can see how other objects are gravitationally effected by it.  Hints the above picture.

Above is a black hole that is gaining mass (accretion) from a star. (That black hole is a thief.....It's stealing all the stars mass...lol!)

            Black holes are the least common final stage of stellar evolution and the most unexplained stage.  Black holes are a remnant of massive stars around 10-15 solar masses and are formed from the cores of the massive stars after a supernova.  The remnant of 3.2 solar masses or above is left over after the supernova (2). Both electron and neutron degeneracy pressure are not enough to balance the remnants gravity so it collapses.  This collapse produces such a sophisticated entity that not light can even escape it after the event horizon is passed.   

            Black holes, very massive ones, are found at the center of galaxies. The black hole at the center of the Milky Way is about 4.3 million solar masses. There are many black holes bigger than the one found at the center of the Milky Way and there are also smaller ones.  Black holes do not start of very massive (over 8 solar masses), they usually gain matter from objects failing onto them and or they merge with other black holes.  Stars can be tidally disrupted by super massive black holes (5).  When a star passes close enough to a black hole, the black hole can ‘steal’ mass from the star.  This is a very interesting phenomenon and many physicists and astronomers have researched it. 

            The universe is very big and it is comprised of many different objects.  Stars, planets, comets, asteroids, dust, and many other objects populate the universe.  Stars are the most researched part of the universe because there are billion and billions of stars.  Each star depending on its initial mass will either end up as a white dwarf, neutron star, or black hole. 

  

1. Neutron Stars and Pulsars. December 2006. Goddard Space Flight Center. http://imagine.gsfc.nasa.gov/docs/science/know_l1/pulsars.html
2. Black Holes.  December 2010. Goddard Space Flight Center. http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html
3. White dwarfs. December 2006. Goddard Space Flight Center. http://imagine.gsfc.nasa.gov/docs/science/know_l1/dwarfs.html
4. Massive Mega-Star Challenges Black hole Theories. Space.com.  Clara Moskowits. December 18, 2010. http://www.space.com/8970-massive-mega-star-challenges-black-hole-theories.html
5. Tidal disruptions from Black holes. Colloquia and class. September 13, 2012. Enrico Ramirez-Ruiz.

Life Cycles of Stars

Lets star out by saying most stars begin there lives in a molecular cloud.  At some point in this cloud when the big "ball" of matter comes together, the matter begins to heat and then it begins to collapse.  As it collapses the cloud becomes more masses because it gains matter from the surrounding accretion disk. 

In the above photo you see the protostar (ball of matter) the bright spot in the middle, and the accretion disk surrounding it.

Ok, now here is the good part. The protostar after it has gained enough mass and is hot enough it will begin nuclear fusion in its core.  The easiest example I can give to explain what nuclear fusion is, is when a star converts hydrogen into helium in its core.

If a star is larger than 200 times the mass of the sun it will not become a star. In simplest terms it blew itself up....lol! Also a star less than 0.1 times the mass of the sun will not become a star because it cannot reach high enough forces to begin nuclear fusion in its core.

So if a star is above .1 solar masses and below 200 solar masses, what happens next? It becomes what we know as a main sequence stars.  Main sequence star burn hydrogen into helium in the core.  Depending on how massive the star is it can take billions years to burn through its hydrogen or it could take millions of years. A great example of this is the H-R diagram pictured below.  The stars that are in the like line that looks like a negative slope are those of the main sequence.

The higher the mass (the hotter the star) of the star the faster the star will go through its life cycle.

After stars go through their main sequence stage, they have what we call a helium flash, the outer layers expand and the core contracts.  The star is now exponentially bigger and it is red.  I can use our sun as an example. In 4.5 billion years our sun will complete its main sequence stage and turn into a red giant. When it does everyone will burn to a crisp! The sun will become so big that it will engulf the planet.  So it you are around then please have an escape plan (also look out for other stars from Andromeda because our galaxies will be merging around that time too.).  Oh and if you can get away please take pictures and tag me on facebook!

So after the red giant phase, then it gets a little tricky. For stars around the same size as our sun they will become a planetary nebula they eventually end up as a white dwarf(I will do a post of deaths of stars). 

For stars much much more massive than our sun, the stars will go supernova and blow up and later become a neutron star or black hole.

Star Formation

There are billions and billions of stars in our Universe, but a big question that many people ask is were do these stars come from and how are they formed. Well I am about to give some general information on star formation.

Stars form in molecular clouds, also refereed to as star nursery's because star formation occurs within them, that are dense enough to allow the formation of molecules. The most common molecule being Hydrogen seeing as how hydrogen makes up about 75%, of the universe's baryonic (ordinary) matter.

Molecular Cloud:

In these clouds turbulence or fluctuations make it so a certain amount of matter in the cloud can join together then gas and dust in the cloud begin to collapse under its gravitational attraction.  As it begins its collapse, the material in the center starts to get hotter.  This core of hot material is what we call a protostar.

Protostar:

A protostar cannot be called an "official" star until it begins nuclear fusion in its core.   Nuclear fusion begins when a protostar begins converting hydrogen into helium.

Stay tuned for the life cycle of a stars!!

On September 27, 2012 Jeff Oishi joined the University of Michigan Astronomy department to present his data on accretion disks.

Jeff Oishi is a theorist with a broad interest in Astrophysical fluid dynamics. I know that sounds very complicated but in simpler terms he likes to research the natural ways that fluids flow and move in terms of astronomical objects.

He received his Bachelor of Science in Applied Physics from Columbia University in 2000. In 2007, Oishi received his PhD in Astronomy from the University of Virginia. 

 If you want more information on Oishi here is a link to a site with some of his research  http://cms.jsoishi.org/node/3

Oishi presented his research on black holes this past Thursday to the University of Michigan astronomy department.  He seemed to be particularly focused on accretion disks and how they influenced star formation (I WILL DO A POST ON STAR FORMATION AND ONE ON STAR TYPES SO YOU CAN LOOK THERE FOR INFORMATION ON STARS.)  An accretion disk is a structure that is formed by diffuse material (gas and dust) that is in orbital motion (the way the earth rotates around the sun) around a central body. In Oishi's case the central body was a protostar. 

Here are some images of accretion disks:

These accretion disks (the dust inside them), according to Oishi, rotate around the protostar just as the earth rotates around the sun. But, some of the disks matter actually can fall onto the protostar, thus making it bigger and more massive. 

Oishi researches the material in the accretion disks and how that material behaves. He said that he studied the composition of the disks. The density of the disks.  He also studies the fluctuations in the disk.  He says that all of the matter in the disk is not setup in a homogenous way.  The matter can be unevenly distributed throughout the accretion disk. 

Here is an image of a magnetic field in an accretion disk:

All in all it was a great presentation that was very interesting.

The 16th annual Astronomy at the beach was hosted this past Saturday (9/22).  It was hosted by Kensington Metropark and the Great Lakes Association of Astronomy Clubs (GLAAC). It was fun and exciting. At Kensingston park hundreds of people gathered to come out and participate. There were telescopes lined up all around the lake, there were 3D presentations, and there was a keynote speaker. 

Let me start by saying that even though it was an extremely cold outside and that it is totally probably that I caught a cold during that night......It was all worth it.  The speaker was an astronaut. A real astronaut.  He came in wearing a blue NASA jump suit and I even got to shake his hand!!!!

The keynote speaker was Dr. Andrew J. Feustel.  He is a geophysicist from Lake Orion, Michigan.  He went to Oakland Community College and Purdue University. He participated in two NASA missions in 2006 and 2009. Both times his family could not see him land. The first time was because the weather conditions didn't permit it and the second was because it was night when the space shuttle landed.  Feutsel spoke about both his missions and he showed video of himself and the space ship crews in space. I have to say that the videos they made, made space travel look sooooo fun! I guess the fun of making the videos made up for the intense raining he had to do in order to get ready for the missions.  Also he told us that he and his crew members assembled the final parts of the international space station and all i could think of was wow I shook hands with someone that will go down in history!

Here is a picture of Dr. Andrew Feutsel, he was the perfect speaker for Astronomy at the beach and he made a cold night awesome. Google him for more information or use this link http://en.wikipedia.org/wiki/Andrew_J._Feustel. He seemed like a pretty awesome guy.

September 20, 2012 Colloquia

On Thursday September 20, 2012 the astronomy department had an exciting and engaging Colloquium. Wendy Freedman, presented data on two projects she participated in. The first was the CSP and the second the CHP. 

Wendy Freedman has been a frequent guest speaker at the University of Michigan she comes on average once every three years.  Freedman graduated from the University of Toronto in 1984 with her Ph.D. She now works at Carnegie and in 2009 she won the Gruber Cosmology Prize.

She spoke on her and her colleagues work on the Carnegie Supernova and Hubble Constant Project (hints the CSP and CHP). She focused mostly on the current cosmological model, dark energy, Hubble's Constant (Ho) and the Carnegie Supernova project.

She gave detailed information about the Friedman equation and the cosmological framework (Einstein's tension vs. energy momentum tension)

Freedman showed that the current concordance model states that Ho = 72 + or - 5 km/sec/Mpc, but according to her and her colleagues work at Carnegie the newly estimated value of Ho = 73.8 + or - 2.4 km/sec/Mpc. 

This is a huge break through and she is going to study further into it with the CSP-I which began in 2011 and will run for seven years in total according to her. So be on the look out for updated values of Ho and keep up to date on how the CSP project is progressing. 

On September 13, 2012 the Astronomy department had an interesting visitor Enrico Ramirez-Ruiz (pictured below).  Ramirez-Ruiz is a member of one of the best astronomy research schools in the United States, the University of California.

Ramirez-Ruiz gave a very interesting and exciting hour long lecture on the tidal disruption of stars by massive black holes.  He presented his data in a convincing matter which included; pictures, videos, equations, and graphs.

Ramirez-Ruiz said his motivation was that he wanted to unveil dead quasars.  To achieve his goal he said he had to first digest the matter surrounding black holes. This matter of course included stars.  He explained that the orbits of stars can be greatly impacted by massive black holes, he also stated that there were close and long distance encounters.  Ramirez-Ruiz said that stars that are tidally altered by massive black holes engage in a random walk and they have non linear angular momentum. One of the most interesting things he did was compare the equations of both the Schwarzchild radius and the Tidal radius.  He explained that Tidal radius gets weaker with mass gain and that the Schwarzchild radius was linear with mass gain.  The biggest topic he discussed that caught my attention was 'imposter' stars.  He explained how lower mass stars can appear to have greater masses, hints the title 'imposter' stars.

Enrico Ramirez-Ruiz presented an incredible lecture and I felt like it was a true pleasure to meet him and ask him questions about his research. He has an awesome personality and he is quite funny!