Ask An Astronomer

[Lord Chrusher]

I feel the same way about the sky. I find the idea of falling up to be silly.

Lord Hosk, if terraforming is allowed I will be cheeky and say that Mars is easily less than a month away. When you say how long do you mean in the spaceship's rest frame or in the Earth's? If the spaceship is travelling at a large fraction of the speed of light these will be quite different. I would guess about a century would be required along with several times humanity's total power output.

[Lord Chrusher]

Cross posting from the SCIENCE! thread:

My paper on globular cluster metallicities was presented by Astrobites:

http://astrobites.com/2012/07/31/calcium-and-color-measurements-of-bimodality-in-globular-cluster-metallicities/

Run by grad students, Astrobites selects a new, interesting astronomy paper each day and explains it at the level of an undergrad in the physical sciences.

[plummeting_sloth]

Oh wow... this is still a thing. Okay, dear Dr. Star Science! I am going to have a fair amount of night time available to me the last two months of the year. What is some cool shit I can look at in the night sky at that time? I live in the Mid-Atlanic of the US.

[Lord Chrusher]

This is a good source:
http://www.skymaps.com/downloads.html
This is sky in the northern hemisphere for October:
http://www.skymaps.com/skymaps/tesmn1210.pdf

[plummeting_sloth]

Awesome! Thank you!

[Master Gunner]

So, I've been reading Larp Trek, which got me to reading up on the layout of the Bajor system (from Memory Alpha, sourcing from the show and Star Trek: Star Charts). This is what I found, in order from innermost to outermost planet (all of which are named Bajor N, because why not?):
4 small Mercury-type planets
Venus-type planet
Earth-sized Marginally inhabitable planet
Unknown-type planet with three moons
Mars-type planet (with some colonies)
Super-Jupiter Gas Giant
Denorios Belt/Wormhole/DS9
Super-Jupiter Gas Giant
Bajor itself
3 Pluto-type planets

The rough sizes of the planets shown on diagrams in-show and what's listed in the Star Charts book don't quite line up, however. The diagrams would swap the mars type and inner gas giant, and Bajor and the outer gas giant.

So, given that the Bajoran sun is supposed to be roughly the same size as Sol, and Bajor the same size as Earth...is that system remotely possible, or in the most subtle yet epic crossover of all time, was the Bajorian system actually constructed by the Centerpoint station, like Corellia was?

[Lord Chrusher]

Well we call exoplanets things like Keplar-20f and Gliese 581 e so I am not sure we can fault them for calling them things like Bajor XI.

Assuming that the earth-sized planet is on a Venus like orbit and Bajor itself is on an Earth like orbit and the Bajoran sun is a Sun like star I would say it is an unlikely orbital configuration. Having that many massive planets that close together is most likely unstable. I would not want to have Jupiter between us and Venus much less two of them.

I should caution that we are only starting to get an idea what is the typical star system like. We also have only a limited idea how planets form. Also this really is not my area of expertise.

[Lord Hosk]

so planet killing asteroid going to potentially skirt the atmosphere in three years???

[Ptangmatik]

Then loop back and come in even closer in 2036! We're all gonna die.

[My pseudonym is Ix]

Actually, just about the only thing we know for certain about that asteroid is that we aren't going to die; it'll be close, but won't hit.

[AzureAngel17]

The Jet Propulsion Lab concurs.

[Lord Hosk]

I have heard both but cant find good data in my search, too much crap. Is there a scientific consensus on The earth is on a slow path towards the sun or away from the sun?

if so is that a constant or variable rate?

if constant what is that rate?

If variable is there a range that is accepted as reasonable?

[Lord Chrusher]

I hope this answers your questions:

The Earth is constantly moving towards and away from the Sun. The Earth orbits the centre of mass of the Earth Sun system in an elliptical orbit. At its closest approach to the Sun, the Earth is 4 999 942 km closer than its farthest approach (3% of the mean distance). This is equivalent to a mean speed of 1140 km per hour. This amount has varied from almost 0% to 10% of the mean distance over the last million years or so due to the gravitational effects of the other planets. The dynamics of the Solar System are chaotic so it is quite difficult to predict the motions of the planets further and further in the future. We do know enough that we can say that the Earth will stay roughly where it is and no planets will collide in next few billion years.

For a two body system tidal forces will act to make the system tidally locked - where the rotation periods and the orbital period are all the same so the same side of each body always faces the other body. The rotational period of the Moon (the length of its 'day') is already the same as its orbital period (the length of its 'year'). The tidal force of the Moon is causing the Earth's rotation to slow (by about 1.7 milliseconds per century) and the Moon to move further away from the Earth by 4 cm per year on average (The Moon is also on an elliptical orbit so it is constantly moving away and towards the Earth).

The same thing should be happening between the Sun and Earth but is much, much weaker. So the Earth is moving incredibly slowly away from the Sun. However, this assumes that the mass and size of the Sun is constant. Currently the Sun is slowly losing mass by fusing hydrogen into helium and through the solar wind. The radius of the Sun is also slowly expanding. As the runs out of fuel both the increase in size and the loss of mass accelerate. By time it reach the tip of red giant branch, one of the stages at the end of a stars life, the Sun will have lost about third of its mass and have a radius about 250 times larger than its current radius (equivalent to 1.2 times the radius of the Earth's orbit). It will also be about 3000 times brighter.

The lower mass of the Sun will cause the orbit of the Earth to shift outwards to about 1.5 times its current radius by the time the Sun reaches the tip of the red giant branch. The rotation of the Sun will slow as it expands. Magnetic braking will cause it to expand further so that its rotational period will be thousands of years rather than it current period of 25 days (at the Sun's equator; at the poles the period is 35 days). Since the length of the Sun's rotation period is now much longer than period of Earth's, the tidal force of the Sun on Earth now acts in the opposite direction and moves the Earth towards the Sun. Since the surface is now much closer to the Earth, the strength of this force is now much, much larger and the Earth spirals in to its fiery doom.

If you want all the gory details about the fate the Earth this is the paper I got the numbers from.

[Lord Hosk]

The same thing should be happening between the Sun and Earth but is much, much weaker. So the Earth is moving incredibly slowly away from the Sun. However, this assumes that the mass and size of the Sun is constant. Currently the Sun is slowly losing mass by fusing hydrogen into helium and through the solar wind. The radius of the Sun is also slowly expanding. As the runs out of fuel both the increase in size and the loss of mass accelerate. By time it reach the tip of red giant branch, one of the stages at the end of a stars life, the Sun will have lost about third of its mass and have a radius about 250 times larger than its current radius (equivalent to 1.2 times the radius of the Earth's orbit). It will also be about 3000 times brighter.

The lower mass of the Sun will cause the orbit of the Earth to shift outwards to about 1.5 times its current radius by the time the Sun reaches the tip of the red giant branch. The rotation of the Sun will slow as it expands. Magnetic braking will cause it to expand further so that its rotational period will be thousands of years rather than it current period of 25 days (at the Sun's equator; at the poles the period is 35 days). Since the length of the Sun's rotation period is now much longer than period of Earth's, the tidal force of the Sun on Earth now acts in the opposite direction and moves the Earth towards the Sun. Since the surface is now much closer to the Earth, the strength of this force is now much, much larger and the Earth spirals in to its fiery doom.

this was more what I was referring to is our orbit moving inward or outward. If I am reading that correctly, our orbit is expanding but the sun is expanding more rapidly. So we are getting further from the central point of the sun, but closer to the surface?

[Lord Chrusher]

Yes. It is not happen very quickly right now - maybe a couple centimetres a year.

[King Kool]

I have a question: can a supernova destroy the galaxy like it was going to do in Star Trek (2009)?

[Master Gunner]

Not an astronomer but...if nothing else, it would take 100,000 years to do so, given the size of the galaxy and the light speed hard limit.

Though of course, there is always the expanded universe to "explain" that.

[Lord Chrusher]

I have a question: can a supernova destroy the galaxy like it was going to do in Star Trek (2009)?

Nope. Supernovae have been going off about a few times a century for more than 13 billion years and our galaxy is still here. This is not to say that supernovae do not have any effect on their host galaxy. They can remove gas from the galaxy, shutting down or slowing star formation.

Master Gunner galaxies are dense enough that the space within them is not expanding.

[Master Gunner]

I meant the Star Trek books and the like, not the expanding universe. :p