Pulsars: The Cosmic Lighthouses

Posted on June 15, 2012 by danielfeldman

When very large stars die, they go out with a huge explosion called a supernova. The outer layers of the star are ejected outwards in a magnificent, extremely bright explosion that can outshine an entire galaxy; the inner core of the star collapses inwards, and if the core is between 1.4 and 3 times the mass of our sun, it forms a neutron star.

Neutron stars are often seen as pulsars. But what is a pulsar? Pulsars are rotating neutron stars that have highly focused jets that point towards us at regular intervals. To explain this type of object and what we see, astronomers like to use a lighthouse analogy:

When you are near a lighthouse at night, you see a sight just like the one in the above link. A beam of light shines out in a specific direction, which rotates at a fixed rate. If you’re close to the lighthouse, you see the light beam regardless of the direction, but what happens when you are far away from the lighthouse? When you are far enough away, you will only see the light from the lighthouse when it is pointing directly at you. And you’ll only see a dot of light when it is pointed at you. So over time, you will see a blinking dot coming from the lighthouse.

The same thing is true with a pulsar. Pulsars can only be seen when the focused beam coming from it is pointed directly at us. So what we observe is a blinking dot of light, or rather a repeating signal. These blips are called pulses (hence the name pulsar).

If a pulsar’s beams never point towards us, we never see it. These are called Pulsar Party Poopers, or PPPs.

Because many pulsars won’t be pointing towards Earth, there are many more that will never be discovered (unless we travel to a different star system that the other pulsars point at).

In general, pulsars rotate really fast (compared to Earth, anyway), but some rotate much faster than others. They can rotate many times each second or faster! Over time, pulsars slow down, so comparing rotation rates can give us a good qualitative estimate of a pulsar’s age.

When pulsars were first discovered, it was thought that they could be alien signals. Don’t laugh–put yourself in the shoes of Jocelyn Bell (and her thesis advisor Antony Hewish) who discovered repeating signals coming in at 1.3 second intervals, much faster than any known stellar rotation rates (and in the radio, no less)! What would you think it was? Eventually it was realized that this was not the case, and the Little Green Men (LGM) theory was put to rest–but Hewish received the Nobel Prize for the discovery (yeah–in typical sexist fashion, Bell was pushed aside and didn’t get her just reward).

To end this entry, some fun facts about pulsars:
1) The Crab Nebula, which resulted from an explosion observed from Earth in 1054 AD, has a pulsar in the center that rotates at a rate of 30.2 times each second.
2) The fastest known pulsar rotates at a rate of 716 times each second!
3) Pulsars have extremely strong magnetic fields, ranging from roughly 1 trillion to multiple quadrillions of times larger than the Earth’s magnetic field.
4) Not all neutron stars are pulsars. All pulsars are neutron stars.

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Venus Transit 2012 – Don’t Miss!

Posted on March 4, 2012 by danielfeldman

Hi everyone! I’m writing this post to alert you to a very rare transit that you MUST see this year–if you don’t, you will never get the chance again. No seriously, you will never get to see it again. I’m of course talking about the transit of Venus across our sun, which will occur on June 6th (June 5th in some places due to the international date line), 2012. This rare event is when we, from Earth, will see Venus crossing in front of the Sun. This event will not occur again until December 11th, 2117. I don’t know about you, but I do not plan on living that long.

So what is a transit? In astronomy, transit actually has multiple meanings, but the one I am referring to, broadly speaking, is when one celestial object passes in front of a much larger one. We say that the smaller celestial object is the one that is transiting the larger one. When this happens, the smaller object blocks only a small fraction of the larger object—if the transiting object blocks most or all of the second one, we no longer call it a transit, we call it an occultation.

Not drawn to scale.

Keep in mind that you should not just look at the Sun during this event—I’m sure you all remember all of your elementary school teachers and parents telling you that if you look directly at the Sun, you’ll go blind. That’s still true, unless you use what’s called a filter. By looking at the Sun using the proper filter, you can block out a significant portion of the light, making it much safer to observe the Sun. The best way you can enjoy this rare event is to trek out to your local observatory or amateur astronomer gathering, where there is likely to be a telescope (or multiple telescopes) equipped with the proper filter. If you do not have access to a local gathering of Astronomy nerds, you can view the Sun safely at home with your own telescope!

Transits and occultations play a role in a lot of awesome Astronomy research, and in cool observable phenomena! For instance, one of the most useful and popular ways of detecting exoplanets is through transits! One of the most popular scientific missions currently being carried out is the Kepler mission: Kepler observes a large number of stars continuously, and is hoping to see an exoplanet transit in front of its host star! Similar to the Venus transit, planets in other solar systems can transit their own stars, which blocks out a small portion of the light coming from the star—Kepler can see this by observing a regular drop in the total light it sees from a star. Kepler is so precise, it can see extremely small drops, allowing it to find planets as small as Earth!

Of the phenomena I mentioned in the last paragraph, the most famous and breathtaking are those of solar and lunar eclipses! For a solar eclipse, the Moon is in the same line of sight as the Sun. However, since from Earth, the Moon appears to be the same size as the Sun, it can completely block out the Sun, causing a solar eclipse (although you still see the corona of the Sun, which is the extremely hot outer layer of the Sun’s atmosphere). For a lunar eclipse, it’s a little different—the Earth passes in front of the Moon-Sun line of sight, and the Moon ends up in the Earth’s shadow. Think of the last time you played with a flashlight—you project the light onto the wall, and then place your hand in front of the flashlight—this creates a shadow on the wall, which often looks like a really screwed up animal. This is because your hand is blocking out some of the flashlight’s light, and so you only see the remaining light on the wall. For a lunar eclipse, the Sun is the flashlight, the Earth is the hand, and the Moon is the lit up piece of the wall, except in a total lunar eclipse, the Earth completely blocks the Sun (the hand fully covering the flashlight). When this happens, you get a lunar eclipse.

I hope I’ve now convinced you that transits are really cool, and that you must do your best to observe this rare, awesome Venus transit on June 6th (or 5th)! Check where you need to be to see it, and what your best viewing options are! You won’t regret it.

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Twitter account!

Posted on March 2, 2012 by danielfeldman

Hi everyone! Just letting y’all know that Science for Dessert is now officially on Twitter! I hope to be posting links to cool science for you to check out, and of course tweet when I have a new post. So feel free to follow me, ask me questions, and suggest ideas for future blog posts. It’s all about learning some fun new stuff, so let’s make it happen!

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Blue Stragglers: Clinging to life?

Posted on February 11, 2012 by danielfeldman

Time to jump back into Astronomy! Stars are, in my eyes, the true staple of astronomical wonder. When we look up at the sky, what’s the first thing that strikes us? Stars. And yet when it comes to public knowledge and discussion, stars are probably one of the least talked about (whereas more intense topics, like black holes, are discussed more often). Well, we know a lot of awesome things about stars, and astronomers have encountered some fascinating problems when studying these big, brilliant, beautiful objects.

One thing that needs to be known is that stars are not all the same: in fact, like human beings, they come in all colors, shapes, and sizes. It may be hard to tell with the naked eye, but next time you look up at Orion, notice that the star in the top left corner looks a lot different than the others: it’s red! This star, called Betelgeuse (yep, you read that correctly) is actually a red giant, a star in its final stages of life. This is just one example of how even without a telescope, you can observe how different some stars are.

All stars, regardless of their size, live out the majority of their life on what’s called the “Main Sequence.” A star’s size and mass have a huge effect on how it will evolve over time, including how long it stays on the Main Sequence, and how it ends its “life.” The largest stars, which are blue in color, live on the order of millions of years; stars in the middle, like the Sun, live on the order of billions of years; stars at the lowest end, red in color, can live up to a trillion years or more (for those keeping score, our Universe is only about 13.6 billion years old, so none of the smallest stars have ever died!). One of the most important graphs in Astronomy is the Hertzsprung-Russell (H-R) Diagram, shown below:

There’s a lot going on in this diagram, but try not to get intimidated. On the y-axis is the luminosity, or brightness of a star, compared to the sun’s brightness (the higher up, the brighter the star). The x-axis is temperature in Kelvin (don’t worry about the unit, just know that the farther left, the hotter the star). As you can see, the main sequence looks a lot like a straight line, and is where stars stay for most of their life. As they get older, they will move to the giant or supergiant part of the graph, and end up either in the white dwarf part of the graph after they die, or off the chart entirely. Keep this behavior in mind for the rest of this post!

One of the more interesting fields in Astronomy is the study of star clusters. Star clusters are large groups of stars which have formed together and whose mutual gravity keeps them close to each other. Because the stars all formed at the same time, we can say that they are all the same age! Therefore, we can age the cluster by placing all of the stars in the cluster onto an H-R diagram! Because the largest, brightest stars die out first, you can age the cluster by seeing the largest stars that have NOT yet died. A little morbid, but very effective.

One of the more interesting unknowns in this field are a group of stars called blue stragglers. As I explained before, very large, blue stars have very short lifetimes, dying very quickly compared to smaller stars. However, in many older star clusters (often several billion years old) we observe these blue stars that should have died out long ago. These stars are called blue stragglers, and it’s not yet known exactly why they exist!

The most likely theory (in my opinion) is that these stars are made through stellar collisions! In these older star clusters, you end up with regions of space with a lot of stars nearby. In such a densely packed environment, it seems plausible that two stars may collide and/or merge together, creating these blue stragglers who appear younger than the surrounding cluster.

There are other models which have been proposed to explain blue stragglers, but the jury is still out. This is just one wondrous example of the types of mysteries astronomers devote their time deciphering! There’s still so much to learn about the Universe, and as a fellow scientist, it’s this kind of meaty problem that I would love to sink my teeth into. If you have any questions or comments, feel free to leave them on this entry! Until next time!

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