Habitable zone of a star

Hi,
habitable zone of a star. Sounds like a comfy place, right? Well it can be. It is at least on (tiny portion of) Earth which is an example of object in habitable zone. Such a „zone“ is important for astronomers, or maybe it’s just important for headlines in newspapers.

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Habitable zone in a Solar System based on luminosity.

Habitable zone is an area around star where we, with quite limited knowledge on this subject, think that life could be. The simplest „definition“ is that it’s the area where satellite (such as planet) would be able to sustain liquid water. We cannot be sure of course if life needs it but it is the case for the one that evolved on Earth.

The true habitable zone is something a bit more complicated. The simplest case of a planet would be one that behaves as a black body, that means that it absorbs all radiation (light for example) regardless of its wavelength. This is immediately just an assumption because such a planet does not exist. Earth just as Uranus or Mercury reflect light, the planet’s albedo describes this. Albedo is an attribute telling us how much object reflects light. 0 means that it is a black body and 1 means that it is white body aka perfect mirror.

There are even more factors that one could consider. For example, when planet has thick atmosphere it can sustain liquid water (and life) even further out from habitable zone on the other hand if that happens to planet like Venus which is already pretty close, you have got hell. If satellite orbits with high eccentricity the conditions are again different.

It’s hard to combine all of this together which results in lot of different outcomes depending what model one picks. Estimates for Solar System are between 0.9 or even 0.6 to 1.3, 2 or 3 astronomical units. In most of them Earth is just on the inner edge. These numbers were pulled from Wikipedia.

When we hear in news that a new exoplanet was found in a habitable zone it might not mean much. This news usually come alongside the information that the planet has similar size that of Earth, it’s not like we could travel there or anything, now we are mostly collecting data and learning.

Dragallur

HZ picture: By Habitable_zone-en.svg: Chewiederivative work: Ignacio javier igjav (talk) – Habitable_zone-en.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8462897

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How do we know that Sun is a star?

Hi,
today I will shortly explain how is it that we know that Sun is a star and why it is not so trivial.

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When you look up on the clear sky in night you see dots and they are said to be basically the same thing as our close Sun. They definitely shine, but not so much, they are not very warm, light up in different time than Sun, for some reason twinkle and are way smaller, even with binoculars they are still dots.

Hell yeah, they are quite small dots! Even if you take our best telescopes you will always have them so tiny! They are “point source of light”, which means that they are simply so small that from our view and practical purposes they are dimensionless.

They are huge though, most a bit smaller then Sun but still big, but so far away that they seem only as points. If even now we can not see the star’s surface as anything else than a dot, how do we know that it is the same thing as Sun?

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The thought of Sun being star was there for some time. I found that already guys in ancient Greece thought that, the same idea came to the famous row of astronomers like Copernicus, Galileo, Kepler, Newton and others but they still did not have an access knowledge that would prove it.

In 1838 Friedrich Bessel measured the distance to star for a first time without considering anything about it (I guess he might have used stellar paralax but I am not sure). He found that the distance is huge, as we know today and soon we calculated that these dots are actually about as bright as Sun, also with astronomical spectroscopy scientists found out that what is happening there is also happening here. To make it clear, we do not have a “close up” image of any star, nor do we have image of any exoplanet.. for now that is simply impossible, still we can admire the cool Jupiter that Juno is taking pics of! (it is literally porn for eyes 😉 )

Dragallur

Why do stars twinkle (and planets not)?

Hi,
I felt so embarassed that I finally had to find it out and now I am writing this short post about it. For few years, roughly, I am studying astronomy yet, I never knew why stars twinkle and planets not. I confess.

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Stars twinkle because the light that reaches us goes through atmosphere and atmosphere is not very homogenous – smooth. Air refracts light and there is different temperature once in a while, humidity and so on, I think that lot of factors play the role. This causes the light of star to scatter a bit and creates the twinkling effect.

Planets do not do it. This is great because you can identify them extremely fast on the sky and you do not mistake them for some other bright star. Why? Their light still goes through atmosphere. Because they are not “point sources”. Stars are so far away that even with best telescopes we see them only as points. Planets with simple telescope on backyard already have shape. Some of their light scatters one direction, some the other and it basically cancels out creating nice image. This is also why it is better to go star-gazing in the winter, colder air does not create so much “noise” on the picture.

Dragallur

Standard candle

Hi,
have you ever wondered how do scientists measure distances far away in space? Maybe you have heard about supernova which was found about 11 billions of light years away. The problem is that when something is far away you don`t actually know if what you see is bright or close or dim or far away or the combination of both.

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So here comes the standard candle. The standard candle is object in Universe which has always about the same luminosity.
The example of thing that is not standard candle is for example any star since when you are looking at it you don`t know its luminosity since it could be in spectral class O or K  and it could be next to you or far away.

Standard candles are supernovas of type Ia which are the brightest of all supernovas.
First of all I will try to explain what this supernova type Ia is.

This type of supernova is explosion of white dwarf which exceeds the mass of 1.4 the mass of Sun.

How this happens? This white dwarf has to be in binary star system so he can feed on his companion`s gases and get the mass again to trigger fusion. When white dwarf exceeds the Chandrasekhar limit which is 1.4 of Sun`s mass as mentioned above then it explodes in violent explosion called supernova Ia.
Actually it is not so simple because the supernova explodes in what is called “double detonation” which is caused by first explosion happening even before Chandrasekhar limit because of the hydrogen which is fused on the surface on helium (I am going to find out more about this on astronomy stack exchange so I will update it.)

Then the second and main explosion is triggered and the donor, the first star is thrown away from the system by the energy released (1-2*10^44 J).

So the important thing is that supernova works as standard candle because it will be always about the same brightness which is very important because than astronomers can calculate how far away it is because they know how bright this thing is.
Supenovas are used for distances greater than 1,000,000 light years because closer there is not enough of them.

Dragallur

Picture source

 

Debris disk: Ring around star

Hi,
with this post I am answering to one comment posted about three weeks ago on Roche limit.

Could there be rings around sun?

Yes they are called Debris disks. Debris disk is created from nebula which is surrounding young star. This nebula is full of gas and dust. Either all this stuff is pushed away by radiation or planetesimals are created.

Planetesimals are huge rocks, (more than one kilometer across). They do not need Brownian motion anymore to grow because they have enough strong gravity to pull other rocks, and at some point they can be enough large to be planets.

When those planetesimals collide they left dust and rock behind them. This is what is called debris disk. It is not created by Roche limit as normal planetary ring is and I did not find anything about rings around star that are made by Roche but if I do I will definitely post it here.

This dust in debris disk is usually very small: 1-100 micrometers.
Our sun has this debris disk and you have probably heard about it, it is the Kuiper belt.

There is other example of debris disk and it is around very bright star called Fomalhaut. You can see it on the right top. This star has debris disk which is moved by 1.4 billion miles (roughly 2.24 billion kilometers) to left. It is probably because of huge planet. The dark spot around star (pic. on the left and bot.) is just effect of blocking the light from Fomalhaut so we can see all the debris reflecting photons. 
Fomalhaut has at least one planet and this one is called Fomalhaut B. On wiki, I am pretty sure there is mistake in one sentence since they are saying that it is star.
You can easily find Fomalhaut on Celestia, I will write about that program probably next time.

Dragallur
PS: I lost.

Tidal forces: Roche limit

Hi,
thanks to one of my frequent readers I will add short post to tidal forces because I forgot very important thing: Roche limit.

Roche limit is distance from object (star, planet). If satellite (moon) crosses this distance it will be eventually ripped apart (by tidal forces). Important is that for every satellite it is different because every satellite, like man-made has different properties. It is very complicated to know exact Roche limit for anything because there is lot of stuff you must count in.

1)2)
3)4)

On first picture there is planet orbiting sun in save distance and no great tidal forces are disturbing it.

On second planet is coming closer to Roche limit and its shape is disturbed because red side is much closer.

When planet crosses limit is starts to be ripped apart. Stuff nearer (red) starts to fly faster and that is end to our planet.

Dragallur

Civilizations: Dyson´s sphere

Hi,
Again I will write about civilizations but this time only about Dyson´s spheres which I mentioned before as a thing which would be used by civilization of type II and type III.

Because there is high probability of all civilizations using energy, we assume that all “worlds” will need more and more energy as their population grows.
At one point there wont be enough energy on its home planet. They/we will need some source of energy which would be kind of permanent. Here comes this Dyson´s sphere because it is a sphere which surrounds home star and collects its energy.

First there are smaller types of Dyson´s sphere like a Dyson´s ring. It is ring of objects sharing the same orbit and collecting energy of star. They would transfer the energy by wireless power transfer.
Expansion to ring is swarm which is lot of rings around star. BUT those ring´s orbit would intercept each other so it would be really difficult to avoid some crush.
To solve this you would have to have statellites instead of satellites. Those would be objects which would be stationary and they would not orbit the star. Those things would fight with gravity by large sail which would be pushed away from the star by solar wind. BUT this sail would have to be extremely light.. when I say extremely I mean very extremely because paper has 80g for squared meter and this sail would have to be about 0.7g for m^2. This would require ultra light nanotubes.
Last type is sphere which would collect almost all energy made by star. It would be extremely difficult to stabilize it and collect all the material.

Dragallur