Galaxies!

Did you know that there are several different types of galaxies? Well, there are. Spiral Galaxies, Barred Spiral Galaxies, Elliptical Galaxies, and Irregular Galaxies all exist in the cosmos.

Spiral Galaxies, like our own Milky Way, are characterized by arched "lanes" of stars. Spiral galaxies generally have solar masses anywhere from 10^9 to Normal 0 false false false MicrosoftInternetExplorer4 4x10^11 and solar luminosities of 10^8 to 2x10^10. Spiral galaxies are further classified according to the "texture of the spiral arms and the relative size of the central bulge.

Similar to Spiral galaxies are Barred spiral galaxies. The spiral arms originate at the ends of bar-shaped regions running through a galaxy's nuclues. A barred spiral galaxies have similar solar masses and solar luminosities as regular spiral galaxies and are subdivided by the size of the central bulge and characteristics of the spiral arms.

Elliptical galaxies, surprisingly enough, have distinctively elliptical shapes and no spiral arms. They contain almost no interstellar gas and dust, and so almost no new satr formation takes place in them. Elliptical galaxies have solar masses of about 105 to 10^13 and solar luminosities of anywhere from 3x10^5 to 10^11.


Irregular galaxies are galaxies that don't fit into another category. They are usually rich in interstellar gas and dust and contain both old and new stars. Irregular galaxies have solar masses anywhere from 10^8 to 3x10^{^10} and solar luminosities anywhere from 10^7 to 10^9.

The Galactic Center!

Our galaxy is a pretty cool place. It may not be anything special, but it's home and it's ours, and that's good enough for me. Even though the Milky Way is pretty normal, there are still some pretty strange things going on.

Take, for example, our galactic center. As one might expect, it's a pretty crowded, busy space: there are lots of stars that interact with each other often. In fact, if you lived on a planet close to the galactic center, the total intensity of light from stars would be the equivalent of 200 of our full moons. Basically, night as we know it, would never occur. Amid all this brightness, however, is the darkest object that can exist, as far as we know: a supermassive black hole, millions of times more massive than our Sun.

Sagittarius A* (you say, "A star"), as far as we can tell, is very, very close to the galactic center. Hundreds of stars crowd within one light year of Sagittarius A* (in most of the galaxy, the average distance from from star to star is one light year). Sagittarius A* doesn't actually appear in infrared images. Scientists, however, have been using infrared observations to make alarming and astonishing discoveries.

Scientists have found a large number of stars orbiting around Sagittarius A* at speeds of over 1500 km/s (for comparison, the Earth orbits the Sun at 30 km/s)! It has also been found that stars have incredibly small elliptical orbits, within 45 AU, or about 1.5 times the distance from the Sun to Neptune.

Sagittarius A* must exert an incredibly powerful gravitational force to keep stars in such small, rapid orbits. Using Newton's form of Kepler's third law to stars that orbit Sagittarius A*, scientists have discovered that whatever it is, Sagittarius A* would have to be 3.7 million solar masses.

Sagittarius A* can only be one thing: a supermassive black hole.


Cue Muse's "Supermassive Black Hole." It doesn't have anything to do with Astronomy, but it is damn catchy.

Gravitational Lensing!

I'm not going to lie, I don't understand a lot of Einstein's model of General and Special Relativity. One thing, however, I do understand (well, I at least understand the concept of it, I still have a hard time with wrapping my head around spacetime as an actual thing). Anyway, Gravitational lensing is one of the few things this semester I got the concept of immediately.

Gravitational lensing was one of the first tests that helped to prove Einstein's theory of General Relativity. First, it's important to realize that the "fabric" of spacetime is curved by objects with mass, but objects with larger masses curve spacetime more. The Earth curves space time, but the Sun curves it more.



In class, we used the anatomy of a water bed. The surface of the water bed is spacetime, and if you put a small object on the surface, it creates a small dip around the object. Bigger object (more massive object), bigger dip.

Got that? Good. Moving on.

Light rays travel in straight lines, right? Well, not necessarily, all the time. If the spacetime through which a light ray is traveling is curved (like when light passes near the surface of the Sun) the path of the light ray is also curved. Basically, gravity bends light.


The maximum light will bend is only 1.75 arcseconds, but it is enough to make far-away stars appear to be in a different position then they actually are. Another effect of gravitational lensing is that very, very distant objects seem brighter, and are therefore easier to see and study.

Nebulae!

It's only my opinion, but stellar nebulae are some of the most beautiful occurrences in astronomy. There are several different kinds of nebulae including Emission Nebulae, Dark Nebulae, and Planetary Nebulae.

Emission nebulae are found near very hot, very bright stars (usually O and B stars). They occur when O and B stars give off large amounts of ultraviolet radiation. When cold atoms of gas in the interstellar medium absorb the ultraviolet radiation, an emission nebula is formed!


A Reflection nebula is a bluish haze that surrounds a star. They are caused by very fine grains of dust around new stars. The light we see coming from the nebula is starlight that has been scattered and reflected by the dust grains.


A Dark nebula is an area that is so densely packed with dust that it literally blocks out all light from behind it. The dust scatters and absorbs light very efficiently. Dark nebulae are cool enough (10K to 100K) for hydrogen atoms to form molecules.


Planetary nebulae, unlike the other types of nebula, occur when a star is at the end of its life span. Low-mass stars (like the Sun) gently expel its outer layers into space when it dies. The ejected gas forms a glowing cloud (the planetary nebula).

Women in Astronomy!

The really cool thing about Astronomy is that women have made some of the most important contributions to the field.

For example, a team of women, led by Willamina Fleming, refined the original spectral classification system. Originally, it was classified A through O according to the hydrogen Balmer lines in its' spectrum. This system did not explain why the Balmer lines in some stars were were stronger than the lines in others. There seemed to be no rhyme or reason for it.

Because of the efforts of a team of women in 1897, including Williamina Fleming, Antonia Maury and Annie Jump Cannon, a new spectral classification was born that changed smoothly from one spectral class to the next. Many of the A through O classes were dropped and the ones remaining were reordered (OBAFGKM).

It was later discovered that these spectral classifications placed stars in order of temperature. O stars, the hottest, were at the top, while M stars, the coolest, were placed at the bottom.

Pickering's Harem

Women astronomers hard at work
Williamina Fleming, standing center

How Many Licks Does It Take To Get To The Center Of The Sun? The World May Never Know.

Much like a gobstopper, the sun has several layers: the core, the radiation zone, the convection zone, the photosphere, the chromosphere, and the corona.

Thermonuclear fusion (a process that involves several steps, including turning Hydrogen into Helium, and ultimately gives off energy in the form of gamma rays plus a few neutrinos) occurs in the core. The core is the only place in the Sun that has enough energy (that is hot enough) for thermonuclear fusion to occur.

In the radiation zone, energy created through thermonuclear fusion in the core, travels further and further outward. Astronomers believe that energy travels through the radiation zone as electromagnetic radiation. Astronomers also believe that the radiation zone is so dense, that it could take upwards of 171,000 years to escape into the convection zone.

After energy has passed through the radiation zone, it reaches the convection zone, the final layer in the Sun's structure. In this layer, energy is usually transported using convection. In the Sun, the convection zone takes up the outer 30%.

After leaving the convection zone, energy makes it's way to the Sun's atmosphere. The layer closest to the convection zone is called the photosphere. The photosphere is primarily made up of Hydrogen and Helium, the two most abundant elements in the solar system. The photosphere is not very thick, compared to the rest of the sun, but we can only see approximately 400 kilometers into it. Were it not for the opaqueness of the photosphere, theoretically, we would be able to see into the sun's interior for thousands of kilometers. Solar flares and sunspots take place in the photosphere.

The chromosphere, usually washed out because of the incredibly bright photosphere, is only visible with the human eye for mere seconds during a total solar eclipse as the moon covers the edge of the photosphere. Arguably the most spectacular of solar phenomenon, solar prominences are made of plasma around the same temperature as the chromosphere (around 10,000K).

The corona, the outermost layer of the Sun's atmosphere, begins on top of the chromosphere. Incredibly hot (somewhere between 1 and 3 million Kelvins), the corona is most visible to the human eye during a total solar eclipse.