Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 13/07/2012 21:30:01

Title: How long would the Earth take to orbit Canis major?
Post by: thedoc on 13/07/2012 21:30:01

Jake Wretberg  asked the Naked Scientists:
   
Hello Naked Scientists!

I was looking at a photo that compares the size of Earth to VY Canis Majoris. I was wondering if it takes the Earth one year to complete a rotation of our Sun, how long would it take if the earth orbited VY Canis Majoris?

Thanks

Jake from Salt Lake City, Utah

What do you think?

Title: Re: How long would the Earth take to orbit Canis major?
Post by: Soul Surfer on 14/07/2012 00:14:08

VY Canis Majoris is a very large red supergiant star with a diameter of about 20 astronomical units.  This is larger than the orbit of Jupiter.  However its mass is estimated to be only around 15 to 35 times the mass of the sun.  This is because although stars have enormous variations in luminosity and size they do not vary a great deal in mass. 

The reason for this is that the rate at which they generate energy in their core through nuclear fusion increases rapidly as the mass of the star increases and the size of the star is determined by its mass and thee rate at which it is generating energy so a relatively light star which is generating energy faster towards the end of its life inflates into a red giant to get rid of the energy in the core.  One day in the future our sun will probably be bigger than the current orbit of the earth.

The orbital period of a planet around a star only depends on the mass of the star and its distance from the star not the size of the star or the mass of the planet (assumed to be significantly smaller than the star). 

The earth's orbit would be inside the star and so it would be rapidly slowed down and evaporated by the temperature.  Its theoretical orbital speed would only be dependant on the mass of the star inside the orbit so funnily enough because quite a lot of the star's mass is outside the earth's orbit it would probably not be all that much different from what it is around the sun.

The next planet in the solar system outside the surface of the star is Saturn which takes 30 years to orbit the sun

using the orbital period calculator application  http://www.calctool.org/CALC/phys/astronomy/planet_orbit

for a planet that takes 30 years to orbit the sun if the star's mass was 35times the sun it would take 5 years and if it was 15 times the mass of the sun it would take 7.7 years