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View Full Version : Earth and stars to scale.

2008-Feb-24, 03:16 AM
I found this on youtube while looking at size relationship between our sun and earth. Now I know there are bigger stars than our sun out there, but this short video really brought it home about the size difference.
It's hard to visualise something so much bigger than our own sun when you know how big it is anyway compared to earth.
I thought I'd add a question as well. If the habitable band for sol is around the 93 million mile mark, I wonder what it is for the largest star shown here - VV Cephei - if it could support life that is.

G O R T
2008-Feb-24, 10:16 AM
VV Cephei A is a red supergiant that is half a million times more luminous than the Sun. This should move the habitable area out to around 65 billion miles.

filrabat
2008-Feb-25, 12:24 AM
I thought I'd add a question as well. If the habitable band for sol is around the 93 million mile mark, I wonder what it is for the largest star shown here - VV Cephei - if it could support life that is.

Anybody, correct me if I'm wrong on any count. But here it goes.

As near as I can tell, the key to finding out where the life zone lies is in the planet's "Solar Constant" (i.e. Watts per Square Meter Per Second...that is, how many watts of sunlight hit a square meter every second. Earth's solar constant averages 1368 W/m^2/s)*

The formula is: (Luminosity/AU^2); where a Luminosity of 1 = Sun's Luminosity; an AU of 1 = the Earth-to-Sun Distance

Example: Venus's solar constant = 1/0.723^2, or about 1.96; Mars's solar constant is 1/1.4^2, or about 0.44. Multiplying 1.96 or 0.44 by 1368 gives you the SC in W/m^2/s.

*There's A LOT of other factors involved too. The 1368 figure is only for the parts of the earth's orbit in the vacuum of space. The atmosphere and the Earth's intrinsic temperature (i.e. its "blackbody temperature") really complicate things. For now, it's enough to say that atmospheric composition and thickness do play some (but not a HIGHLY drastic) role in determining the life zone location.

2008-Feb-25, 12:17 PM
Thanks for your replies. If the planet was orbiting at the same speed as earth with tilt, I make that just under 690 years to complete one orbit, with each season lasting an average of 172 years.
I'd hope to be born at the start of summer!

KKHS
2008-Feb-25, 04:26 PM
I wonder what it is for the largest star shown here - VV Cephei - if it could support life that is.

It's likely, there isn't one. Not only is there temperature as a factor, but also gravity. These large hypergiants, aren't that much bigger than the, by mass, as they are by diameter. So I'm not sure if there is enough gravity to keep planet in orbit, in the "life zone". Of cource it's always possible.

AndreasJ
2008-Feb-26, 02:04 PM
The HZ of VV Cephei would be some hundreds of AU distant, which is easily close enough for a stable orbit around the star. For comparison, Sedna's orbit has a semimajor axis of 526 AU.

The other question is whether it's distant enough: VV Cephei A and B have a separation of 17 to 34 AU, and I don't know how far out a planetary orbit about the centre of mass has to be to be stable.

Cougar
2008-Feb-28, 03:10 AM
A little something from Kaler's Extreme Stars:

To be properly warmed by the dim red dwarf star LHS 2924, we would have to be only a million km away and our year would be less than an Earth day. Now replace LHS 2924 with Deneb, which (bolometrically) is four billion times brighter. To receive the same amount of heat we do now, the Earth would have to orbit at a distance of 450 A.U., 15 times farther than Neptune is from the Sun. Deneb... has a mass about 25 times that of the Sun.... our hypothetical planet would take 2000 years to orbit... Imagine a winter 500 of our years long, a period that would seem short if we were to orbit a star like Cygnus OB2#12 (http://www.tim-thompson.com/cyg-ob2-12.html).

WalrusLike
2008-Feb-28, 03:48 AM
Does anyone have links to video for showing the relative positions of the planets in terms of distance from sun. When I was a kid we got some marbles and basketballs and beachballs and placed them on our school oval... and at that scale we needed to have pluto in the next suburb.... I want to show my kids something to surprise them like that did for me.

[edit: BTW thanks for the original link... I have emailed it to the kids.]

2008-Feb-28, 05:22 PM
Does anyone have links to video for showing the relative positions of the planets in terms of distance from sun. When I was a kid we got some marbles and basketballs and beachballs and placed them on our school oval... and at that scale we needed to have pluto in the next suburb.... I want to show my kids something to surprise them like that did for me.

[edit: BTW thanks for the original link... I have emailed it to the kids.]

Hope this helps...

This link is probably like the one you did... bless her!

crosscountry
2008-Feb-28, 06:46 PM
I found this on youtube while looking at size relationship between our sun and earth. Now I know there are bigger stars than our sun out there, but this short video really brought it home about the size difference.
It's hard to visualise something so much bigger than our own sun when you know how big it is anyway compared to earth.
I thought I'd add a question as well. If the habitable band for sol is around the 93 million mile mark, I wonder what it is for the largest star shown here - VV Cephei - if it could support life that is.

that music makes my head hurt.

tracer
2008-Feb-28, 09:48 PM
Thanks for your replies. If the planet was orbiting at the same speed as earth with tilt, I make that just under 690 years to complete one orbit, with each season lasting an average of 172 years.

Except it wouldn't be orbiting at the same speed as the Earth.

The orbital period P for a planet orbiting a star of mass M at a distance A is given by the Newton-modified Kepler's Law, as follows:

P = SQRT (A^3 / M).

If the planet is 690 A.U. from VV Cephei, and the VV Cephei is 100 solar masses, then the orbital period would be (690^3 / 100 = ) 3.3 million years.

Your summer would be 800,000 years long....

WalrusLike
2008-Feb-28, 10:50 PM
Hope this helps...

Brilliant... many, many thanks.

With the earlier video I had a lot of fun last night watching the kids get REALLY surprised at how big things were. One mentioned that it 'was really disturbing'. (not really in a bad way...) (anyone who knows our place in the universe feels that way every now and again....) And that video caused me to show the oldest a few other interesting physics ones that I saw there.... never would have seen them if you hadn't posted. Thanks again.

Now I cant wait to show Abi's video to them. BTW does anyone know if I can: firstly save the thing to my pc so It plays fast each time its viewed, and secondly have the video setting in the browser (currently firefox but also have IE) set permanently to 'medium' quality so my slow bandwidth doesn't struggle so much?

2008-Feb-29, 03:03 AM
Except it wouldn't be orbiting at the same speed as the Earth.

The orbital period P for a planet orbiting a star of mass M at a distance A is given by the Newton-modified Kepler's Law, as follows:

P = SQRT (A^3 / M).

If the planet is 690 A.U. from VV Cephei, and the VV Cephei is 100 solar masses, then the orbital period would be (690^3 / 100 = ) 3.3 million years.

Your summer would be 800,000 years long....

Thanks for the update. Cool! Well, not for 800,000 years anyway!