# How Are The Limits of a Star's Habitable Zone Calculated?

#### XSSA

##### New member
I've heard the Sun's habitable zone is approximately from the orbit of Venus to the orbit of Mars.

The sun-like star Beta Comae Berenices has a listed habitable zone from 0.918–1.042 AU even though it has a luminosity of 1.357 ± 0.014[7] L☉. It seems like if a star was more luminous that it's habitable zone would be further away however those distances are nearly the same as the Sun's.
(https://en.m.wikipedia.org/wiki/Beta_Comae_Berenices)

Is the radius of the habitable zone not directly related to the luminosity of a star or is it just more complicated than that?

I was hoping to be able to determine the radius of the habitable zones for these sun-like stars:

https://en.m.wikipedia.org/wiki/HR_511
https://en.m.wikipedia.org/wiki/Beta_Canum_Venaticorum
https://en.m.wikipedia.org/wiki/Sigma_Draconis

#### Michkov

##### Member
Keep in mind that atmosphere and its composition may move the HZ boundaries. Judging by a quick look over the page linked this seems not to be taken into account.

#### dman

##### Active member
One of the means of classifying a star is SPECTRAL CLASS

The Spectral Class is determined by the color and luminosity and at what
bands it emits most of its radiation

Spectral Class ranges from (in decreasing luminosity ) from O, B, A, F, G, K, M
(and sub solar class Y "BROWN DWARF)

Starting at Class O are blue and extremely hot and energetic, B - Blue-white,
A - white, F - white yellow, G- yellow (our sun is G2 class), K-orange, M-red

Inside are subclasses based on size/luminosity from 0 to 8

Most astronomers consider only stars of spectral classes F (> 1 - 1.5 solar mass)
to K (.4 - .8 solar mass) to be habitable . Usually refined as F5 to K5
class stars

ALPHA CENTAURI, closest star system to us, is binary system with ALPHA A
a G2 star (slightly larger than sun) and ALPHA B a K1 (slightly smaller than sun

What brings up second point - ALPHA CENTAURI is a binary system (actually a
triple system with red dwarf) where stars orbit common center of gravity

Binary systems are considered less likely to be habitable because planets in
system would be subject to gravitational effects of other star and thus be in unstable and shifting orbits

#### Linguofreak

##### Well-known member
Binary systems are considered less likely to be habitable because planets in
system would be subject to gravitational effects of other star and thus be in unstable and shifting orbits

Binaries have to be taken on a very case-by-case basis. There are plenty of binaries wide enough that our "traditional" Solar system (that is, everything out to Pluto) could orbit one component without significant disruption, and a fair number of binaries tight enough not to disrupt their habitable zone. Of course, you also have to take into account such factors as eccentricity, which can increase the amount of disruption a start causes to planets around its companion.

#### Thorsten

##### Active member
I've actually been somewhat interested in this question and have started to write a simulation for exoplanet dynamics about a year ago:

http://www.science-and-fiction.org/science/worldbuilder.html

It includes an orbit solver, so you can check whether a planet is halfway stable around a binary, radiation balance, and I've started to include atmosphere effects in a semi-exact manner.

(I've never quite finished it because I got swamped by other issues). But there's a couple of test cases on the page, including the thermal evolution on Mercury (which is driven by quite cool orbital dynamics) and the Greenhouse effect on Earth).

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