Core Collapse Supernova

Crab Nebula Supernova SN1054 remnant

When stars much bigger than our sun reach the end of their life, they often experience huge explosions called supernova. They leave behind beautiful supernova remnants like the ever popular crab nebula (pictured above). But why do these stars die in such violent ways? The answer lies deep inside the star.

Stars spend most of their life fusing Hydrogen in their core, which is what makes them shine so brightly. After a star uses up a good amount of its hydrogen, it can start burning heavier elements such as helium, oxygen, carbon, and silicon. It continually burns heavier elements, creating onion like shells of elements on the core, until it gets to iron. At this point, the core is so hot that the light it releases is able to break apart elements down to its constituent protons, neutrons and electrons, essentially undoing the creation of heavy elements that the star just spent its whole life doing. The pressure in the core that's holding up the now "puffy" star begins to decrease and eventually the star implodes on itself. The in fall of material eventually bounces off the now super dense core, creating a shock wave outward. If the shock wave has enough energy, it will burst through the surface of the star, as what we call a core collapse supernova explosion! It essentially blows the star apart, leaving behind only the super dense core, now called a neutron star.

Image Credit: NASA/HST

Beating Extinction

Last ADYK we discussed extinction due to clouds of dust and gas in outer space. When we view objects like the pillars of creation in visible light, the stars are blocked by the pillars of clouds.  To beat the extinction, astronomers look at the infrared light coming from that region, which is able to pierce through the clouds revealing the hidden stars! Infrared light has a longer wavelength, so it's able to travel further and through more material before it gets absorbed or scattered. This is because most of the gas and dust particles are smaller that the wavelength of infrared light.  The concept is similar to that of radio waves. We use radio waves on Earth to transmit information because the wavelengths are very long, on the order of meters. So for the radio waves, things like people, buildings and trees appear "small" or on about the same size scale as the waves are. Therefore, the waves can travel pretty far before they are disrupted. Shorter waves, like millimeter long waves, wouldn’t travel very far on Earth because they are smaller than the objects they must travel past.  Going back to our pillars of creation, infrared waves emitted by the stars are longer than the size of the gas and dust, thus the light can travel through the clouds. The picture above is taken in infrared light, and we can now see all the stars that were previously hidden behind the clouds!

Image Credit: ESO/VLT

Extinction Astronomy Style

When I hear the word extinction, I think of the dinosaurs and endangered animals. Astronomers have meaning for this term too, and it doesn't involve dinosaurs in space or anything like that!

When astronomers point their telescopes towards the stars, they have to look through not only Earth's atmosphere, but also any gas and dust between us and the star in outer space. Since stars form out of big clouds of mostly Hydrogen gas, astronomers often find themselves looking through thick "space clouds" to try and see stars. These clouds can make the stars appear dim, and sometimes block the light completely! This dimming/blocking of starlight is called "extinction" or "reddening". Stars emit all colors of the rainbow, and even many types of light that we can't see with our eyes. These space clouds tend to preferentially block blue light, making stars appear more red than they actually are. This is why extinction is sometimes called reddening. Above is a picture of the Pillars of Creation taken in visible light with the Hubble Space Telescope. This image shows giant clouds of Hydrogen gas, behind which many stars are forming. We can't see the stars though, because the gas cloud is extincting them and blocking them from our view. Tune in next time to learn how astronomers beat this difficult problem and see these hidden stars!

Image Credit: NASA/HST

Apparent vs. Absolute Magnitude

Last ADYK we discussed the magnitude scale and how astronomers use it to quantify how bright a star is. But there's a little more to this whole magnitude idea. Think about this scenario… The sun is very bright, about -27 magnitude, and very big in the sky. What would the sun look like if I moved it very far away? The sun would still emit the same amount of light, but it would look much smaller and dimmer on the sky. I would no longer say it's magnitude -27, but rather some larger (dimmer) magnitude. In other words, stars that are close are going to appear brighter and therefore have a lower (brighter) magnitude. So how do astronomers correct for this distance bias? They have two different magnitude definitions: apparent and absolute magnitude. Apparent magnitude is the one we discussed last time. It answers the question "how bright does that star appear to be in the sky?". Absolute magnitude corrects for the fact that stars are different distances from Earth, and answers the question "If I assume that all the stars are the same distance from Earth (often assume 10parsecs), how bright does the star appear to be?" Absolute magnitudes allow you to directly compare the light output of two stars without worrying about the fact that they might be different distances away. With some basic algebra, you can switch between the absolute and apparent magnitude of a star, as long as you know how far away it is. Both of these magnitude scales are used by astronomers and are very handy when you are trying to observe or compare the properties of two stars.

Image Credit: http://mrscreath.edublogs.org/2011/12/01/hr-diagram-day-2/

The Magnitude Scale

If you've ever listened to a group of amateur or professional astronomers talk, you've probably heard them say something like: "Yeah, I should be able to image that star, it's magnitude 4." But what does magnitude 4 mean? In astronomy, we use a magnitude scale to define how bright stars and other objects are in the sky. To make it super confusing, the magnitude of a source can be a positive or negative value, and larger positive numbers mean the source is dimmer. You can thank Hipparchus for this, he was the first to catalog the brightness of stars, defining magnitude 1 as the brightest stars in the sky and magnitude 6 as the dimmest. Since then, astronomers have come up with equations to calculate the magnitude of stars, so that the system is not based on how good your eyesight is. The magnitude system is defined such that a difference of 5 magnitudes equals 100 times brighter or dimmer. So how much brighter is star A at mag=2 than star B at mag=3? By definition, 1 magnitude difference equals ~2.5 times as bright, so star A is 2.5 times brighter than star B.

 With today's telescopes, we can see stars that are as dim as about mag 30. Without a telescope, our eyes can't see anything dimmer than magnitude 6. The chart below shows you some common sky objects and how bright they appear. Don't forget, the bigger the number the dimmer the object!

Tidal Locking

You've probably seen a full moon many times during your life, but have you ever noticed that it always looks exactly the same?  The Earth and the moon are tidally locked to each other, which means that the same side of the moon always face Earth. This can happen when you have a small body close to a large body, and gravitational interactions cause the small objects orbit and rotation to synchronize. Let me explain. Intuition tells most people that if you always see the same side of the moon, then the moon must not be rotating on it's axis. But this is not true! You can convince yourself of this by doing a little demo with your hands. Hold up your right hand and make a fist, then point the fingers of your left hand toward it. Now move your left hand around your right, such that the tips of your fingers always point to your right fist. You'll quickly find that you have to rotate your left hand to do that! The moon goes around the Earth once every ~28 days, and it rotates on its axis once every ~28 days as well. This causes the same side of the moon to always face Earth! Planets that are close to their host stars can be tidally locked in the same fashion, and so can two stars. Many moons in our solar system are known to be tidally locked to their host planet, and astronomers speculate that many known exoplanets are tidally locked to their host stars.

RR Lyrae Stars

Globular Cluster M15 which contains RR Lyrae stars

When we hear the word star, we think of a big flaming ball of gas like our sun. Our sun is a "typical" star, but there are many other types of stars in our galaxy. An example of a different type of star is an RR Lyrae star. These stars belong to a class of different types of variable star, or stars that change their brightness periodically over time. RR Lyrae stars are found in globular clusters, and are more aged and contain less heavy metals than the sun. Reactions in the star's core cause the star to physically pulsate periodically in size, temperature and brightness. They change brightness on the order of days. RR Lyrae stars are being studied by many astronomers so that we can understand and accurately predict the brightness changes of these objects. 

Image Credit: Efrain Morales Rivera