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SUPERNOVA: The death explosion of a star that causes the star to shine millions of times brighter than usual.

Star: A ball of mostly hydrogen and helium gas that shines extremely brightly. Our Sun is a star. A star is so massive that its core is extremely dense and hot.

What is a light year?
light travels 669,600,000mph X 24hrs = 16,070,400,000 miles per day X 365 = 5,865,696,000,000 miles per year
5 trillion 865 billion 696 million miles = 1 light year

Ok a better explanation
SOURCE: http://www.howstuffworks.com/question94.htm

A light year is a way of measuring distance. That doesn't make much sense because "light year" contains the word "year," which is normally a unit of time. Even so, light years measure distance.

You are used to measuring distances in either inches/feet/miles or centimeters/meters/kilometers, depending on where you live. You know how long a foot or a meter is -- you are comfortable with these units because you use them every day. Same thing with miles and kilometers -- these are nice, human increments of distance.

When astronomers use their telescopes to look at stars, things are different. The distances are gigantic. For example, the closest star to Earth (besides our sun) is something like 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. That's the closest star. There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn't a practical unit to use because the numbers get too big. No one wants to write or talk about numbers that have 20 digits in them!

So to measure really long distances, people use a unit called a light year. Light travels at 186,000 miles per second (300,000 kilometers per second). Therefore, a light second is 186,000 miles (300,000 kilometers). A light year is the distance that light can travel in a year, or:

186,000 miles/second *60 seconds/minute *60 minutes/hour *24 hours/day *365 days/year = 5,865,696,000,000 miles/year

A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). That's a long way!

Using a light year as a distance measurement has another advantage -- it helps you determine age. Let's say that a star is 1 million light years away. The light from that star has traveled at the speed of light to reach us. Therefore, it has taken the star's light 1 million years to get here, and the light we are seeing was created 1 million years ago. So the star we are seeing is really how the star looked a million years ago, not how it looks today. In the same way, our sun is 8 or so light minutes away. If the sun were to suddenly explode right now, we wouldn't know about it for eight minutes because that is how long it would take for the light of the explosion to get here.

The Question
Is there a possibility that a nearby star could go supernova and destroy the earth? Or have other bad effects on us?

SOURCE: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980521a.html

The Answer
To destroy the Earth itself, the Sun will have to go supernova (which it never will).

If you are talking about the life on Earth, then there is a detailed calculation of the risks due to a nearby supernova on the web:

http://stupendous.rit.edu/richmond/answers/snrisks.txt

The author concludes that a supernova has to be within 10 parsecs (30 light years) or so to be dangerous to life on Earth. This is because the atmosphere shields us from most dangerous radiations. Astronauts in orbit may be in danger if a supernova is within 1000 parsecs or so.

No stars currently within 20 parsecs will go supernova within the next few million years.

There are some indirect effects, though, which are harder to evaluate: the possible effects on the Earth ozone layer is listed in the article above. Additionally, according to one calculation, the neutrino flux from a nearby supernova might heat up the Sun.

Best wishes,
Koji Mukai & Eric Christian
for Ask an Astrophysicist

Proxima Centauri: The Closest Star
What is the closest star to our Sun? It is Proxima Centauri, the nearest member of the Alpha Centauri triple star system. Light takes only 4.22 years to reach us from Proxima Centauri. This small red star was only discovered in 1915 and is only visible through a telescope. Stars of all types from our Milky Way Galaxy are visible in the background. The brightest star in the Alpha Centauri system is quite similar to our Sun, has been known as long as recorded history, and is the third brightest star in the night sky. The Alpha Centauri system is primarily visible from Earth's Southern Hemisphere.

Sirius: The Brightest Star in the Night
Sirius is the brightest star in the night sky. Intrinsically, Sirius is over 20 times brighter than our Sun and over twice as massive. As Sirius is 8.7 light years distant, it is not the closest star system - the Alpha Centauri system holds this distinction. Sirius is called the Dog Star because of its prominence in the constellation of Canis Majoris (Big Dog). In 1862, Sirius was discovered to be a binary star system with a companion star, Sirius B, 10,000 times dimmer than the bright primary, Sirius A. Sirius B was the first white dwarf star discovered, a type of star first understood by Subrahmanyan Chandrasekhar in 1930. While studying Sirius in 1718, Edmond Halley discovered that stars move with respect to each other. There is conflicting evidence that Sirius appeared more red only 2000 years ago.

SOURCE: http://antwrp.gsfc.nasa.gov/apod/ap070427.html

Astronomy Picture of the Day (APOD)

Explanation: One of the brightest galaxies in planet Earth's sky and similar in size to the Milky Way, big, beautiful spiral M81 lies 11.8 million light-years away in the northern constellation Ursa Major. This remarkably deep image of the region reveals details in the bright yellow core, but at the same time follows fainter features along the galaxy's gorgeous blue spiral arms and sweeping dust lanes. Above M81 lies a dwarf companion galaxy, Holmberg IX, sporting a large, pinkish star-forming region near the top. While M81 and Holmberg IX are seen through a foreground of stars in our own Milky Way galaxy, they are also seen here through a much fainter complex of dust clouds. The relatively unexplored clouds are likely only some hundreds of light-years distant and lie high above our galaxy's plane. Scattered through the image, especially at the the right, the dust clouds reflect the combined light of the Milky Way's stars and have been dubbed integrated flux nebulae.