The Sun Today and Every Day

The Solar Dynamics Observatory (SDO) launch in early 2010 and has been studying the sun ever since. SDO focuses on understanding solar storms and the sun's magnetic field, and how all this affects us here on Earth. You can check the SDO website and see what the sun looks like every day! Today we can see a few sun spots and prominences. What will the sun look like tomorrow? We'll have to wait and see!

Image Credit: SDO

Happy Leap Day!

 

 Today, February 29th 2012, is leap day! Every four years an extra day is added to the calendar in February to make up for the fact that one full year is actually ~365.25 days.  But why is it defined this way and what does astronomy have to do with it?

Throughout history, many changes to our calendar have been made so that the seasons and solstices occur on roughly the same dates every year. Since the 16th century we have been using the Gregorian calendar system, which defines one year to be 365days, and one leap year to be 366 days. Every four years we have a leap year, except for years which are divisible by 100 and not divisible by 400. So, for example, the year 2000 was a leap year (divisible  by 100 and 400) but the year 2100 will not be a leap year, because it's not evenly divisible by 400. If you do the math, this results in the average number of days in a Gregorian year to be 365.2425 days. This coincides with amount of time it takes the Earth to go around the sun once (~365.2425 days). This makes sense, but Earth's orbit does not stay in the exact same place in space year after year. This slight shift in Earth's orbit is called precession. This results in a tropical year (the time it takes to go from the exact time of the winter solstice one year to the next) to occur on a 365.24219 day schedule.  So in general thing line up nicely, but actually, we are overestimating  by a tiny amount. If we want the seasons to line up correctly, we will have to make an additional one day correction every ~26,0000 years.  This also doesn't include other astronomical changes to the Earth that occur on even longer timescales, but during your lifetime you shouldn't notice any change between seasons and the dates they occur on.

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