history - Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?

The original Hertzsprung-Russell diagrams constructed by Henry Russell and Eijnar Hertzsprung consisted of absolute magnitude on the y-axis and a spectral type or an indicator of spectral type on the x-axis. Below you can see an original HR diagram produced by Russell in 1913.

When the diagrams were constructed, it was not at all clear what the sequence of spectral types or spectral type indicators actually meant. It turned out of course that the sequence (in modern day parlance O,B,A,F,G,K,M) actually corresponds to decreasing temperature.

Astronomers have simply stuck with this convention to the present day, there is no particular reason for that. Most HR diagrams are now plotted with temperature (decreasing) along the x-axis, although that is not what the original HR diagram was.

saturn - How long do planetary rings last?

I'm surprised that this question hasn't been asked before (here or on Physics), to the best of my knowledge. It's one that I might have asked when I was a bit younger, and one that I think other people will ask.

Anyway, it's clear that Saturn's rings won't form a moon, and the same is likely to be true for other ring systems. However, I'm guessing that they won't last forever (it's just a guess).

How long do planetary rings in general last? What mechanisms could cause them to dissipate/fall apart/end? I'm guessing the Poynting-Robertson effect could come into play, but I'm not sure.

And for anyone curious, yes, I checked just for the fun of it, and Yahoo Answers had a bunch of really, really bad, unsourced and most likely inaccurate answers (given that there was no consensus), ranging from '3 million years' to '13-18 billion years' to 'forever'.

spectra - Are hot stars like O-type stars entirely composed of helium?

The lines that appear in a stars spectrum mainly reflects its temperature not its composition, see here

O-type stars start out with the same sort of composition as other stars, that is they are mainly H and He (approximately 75% and 25% by mass) with traces heavier elements.

telescope - Nebula and galaxies using 70mm scope

Whether you'll be able to see them depends on the levels of light pollution in your area. As TildalWave mentioned, a number nebulae and galaxies are perfectly observable with the naked eye so unless you live somewhere very bright, you should be fine. Under really dark skies, objects like M31 are very easy to find with the naked eye. Where I live, I hardly see it with a bright binocular due to very poor seeing, induced by a large number of shopping centres fond of pointing powerful searchlights at the sky for whatever reason.

As to how much you'll be able to see, you can get a general idea of what you should be able to see in ideal conditions by calculating your telescope's limiting magnitude

In reality, you'll also have to factor in the quality of the optics (mainly transmission), both the telescope and the eyepieces. Of the two you mentioned in your question, you should use the 25 mm one. It will give you a lower magnification, a bigger field of view (better for most bright nebulae and galaxies due to their often significant angular dimensions)

Here's a highly configurable calculator that you can use.

Since you live in the northern hemisphere, objects from the Messier catalogue are great candidates to begin your observations with.

observation - Why are distant objects observed in the near infrared?

I was reading an article that explains why JWST is a successor to Hubble and not a replacement for Hubble. They explained that Hubble's science pushed astronomers to look at longer wavelength. And then they said:

In particular, more distant objects are more highly redshifted, and their light is pushed from the UV and optical into the near-infrared.

So basically to observe the first galaxies, astronomers have to observe in infrared. My question is why distant objects require observations in the infrared?

Is it because they are at a very large distance from us, so the light has lost a lot of energy on its way so it's detectable in the infrared?

the sun - Are we still going to have rainbows if Sun is replaced by another star?

Rainbows would lack most blue, and some green for red stars.
For a blue star, the blue part of the rainbow would be more intense.

For more complex colors, the rainbow may show some gaps. A rainbow is essentially a spectrum of that star light portion, which is visible to our eyes, and to which the atmosphere is transparent.

Stars vary in brightness. A blue giant would be large and glaring, a red dwarf faint.

Colors in the rainbow would be blurred, hence closer to white, for large stars, and sharper, more distinct, for small stars, according to the angular size of the respective light source.

... This all assumes, that there is still rain. With small, red stars, it would get too cold for rain. With large blue stars, Earth would heat up too much.
To adjust for these effects, the distance to the star would need to be modified.
And of course, the length of a year, and the orbital velocity may change.
This could then cause different tides, changes in volcanism, etc.

Exchanging the star could cause various other effects, too, other polar lights, effects to the ionosphere, the ozone layer, atmospheric erosion, more...

big bang theory - The Fermi paradox

I think this too broad, but I'll offer the following:

The star Kepler 444 is orbited by several small, assumed to be rocky, exoplanets. Kepler 444 is estimated to be a very old star, perhaps 11 billion years old, with a metal content of about one third that of the Sun.

Whilst the planets around Kepler 444 are small, they are too hot to be "earth like", but there appears to be no reason why planets at larger orbital radii (that are much harder to detect by the transit technique) should not be there.

Thus the answer appears to be demonstrably, at least 11 billion years ago.

However, the limit cannot be much longer than this, since a certain time must elapse between the formation of the first stars in the Galaxy to the enrichment of the interstellar medium with metals. These metals (all elements heavier than He are referred to as such) are required to build a "rocky" planet. While Kepler 444 demonstrates you don't need a solar metallicity, you still need some.

The fastest place this enrichment took place in general was in the Galactic bulge. A burst of star formation probably enriched the ISM in much less than a billion years.

Thus in principle I would say less than a billion years after Galaxy formation and this is probably not much more than 11 billion years ago.

Kepler 444

http://en.m.wikipedia.org/wiki/Kepler-444

http://adsabs.harvard.edu/abs/2015ApJ...799..170C

The second part of your question is hard. It has taken 4.5 billion years "for life like ours" to evolve. Since we don't fully understand the factors that lead to this, the only realistic answer is that it is probable that it takes another 4.5 billion years after the formation of planets for life "exactly like ours" to emerge.

Space time and aging - Astronomy

Einsteins general theory of relativity explains time dilation caused by gravity-emitting objects. As one experiences more gravity, time will flow slower. That means that "standing" on jupiter, which isn't possible due to the lack of surface, will cause you to move through time faster; But you do not age slower. Suppose your Lifespan is 80 years. On Jupiter you still would live 80 years but the time that has passed on earth during your 80 years would be slightly more than that. The same counts vice versa for the moon.

atmosphere - Why argon instead of another noble gas?

Doing a bit of reading up on this, I might have an answer, though credit where credit is due, the answer isn't really mine:

https://www.reddit.com/r/askscience/comments/3wsy99/why_is_neon_so_rare_on_earth/

When the planets coalesced, it's likely that there was very little ices/gas around the inner planets when they formed and the Earth's atmosphere and water (CH4, NH3, CO2 and H20 being the 4 most common outside the frost line ices). These likely came from asteroids and meteors that formed outside the frost line and later crashed onto earth.

Neon is the 5th most common element in the milky-way but because all noble gases have very low freezing points, it's likely not be very common even on comets or meteors for the same reason that water or CO2 aren't common inside the frost line, Neon, and other noble gases likely stay free and don't collect on comets or meteors in high amounts. (I looked, but couldn't find an article to verify that).

But if comets have low noble gas content, then we have to look for an alternate source. With that in mind, and going back to the first link, Argon is produced by radioactive decay of Potassium 40 and that would explain it's relative abundance compared to the more common noble gas, Neon. Helium (Alpha particles) is also produced inside the earth and Radon is is too in small amounts but Radon also decays - that's not related to your question though.

If Argon on planets comes primarily from Potassium 40, you should expect the amount of Argon to have a roughly similar ratio to the amount of potassium on a planet and not be relative to the percentage of atmosphere. A 2nd factor, how much gets blown off the planet over long periods of time is a factor too. Venus in general should be able to retain much of it's Argon based on atomic weight (40) similar to CO2 (44), but if it loses even a tiny percentage of it's Argon over time, that would be a factor too.

Now, to see if this is possible, I should run some numbers, but I warn you, my math can be a little rusty.

Potassium is the 7th most common element in the Earth's lithosphere at about 0.26% and about 0.0117% of that Potassium is Potassium 40. Using a very rough estimate of 2.3 x 10^19th tonnes for the Earth's crust, 2.3*10^19*2.5*10^-3*1.17*10-4 = about 6.7*10^12 or 6.7 trillion tons of Potassium 40 currently in the Earth's crust. (There's probably a fair bit more in the mantle, so these numbers are rough)

With a half life of about 1.248 billion years, that's sufficient time for over 3 half lives if we start after the late heavy bombardment, which suggests a bit over 7/8ths of the original Potassium 40 in the Earth's crust has decayed into Argon 40, so there should be, given the age of the Earth and abundance of Potassium 40, a bit over 7 times 6.7 trillion tons or, lets ballpark and say a bit over 50 trillion tons of Argon that formed on earth by Potassium decay. (I'm ignoring any that might have been produced prior to the late heavy bombardment, cause I assume that could have blown some of the atmosphere off the earth or heated the atmosphere enough for the sun to blow some of it off). Also, doing a bit of research, only 11% of the Potassium 40 decays in to Argon 40, 89% undergoes beta decay into Calcium 40, so for this to work, there would need to be a fair bit more Potassium in the earth than I estimated, but that's still likely the case.

The mass of the atmosphere is about 5,140 trillion tons, and 1.288% of that (By mass, not volume) = about 66 trillion tons, so the Argon we should expect from Potassium 40 decay and the amount of Argon in the atmosphere are pretty close. Some Argon gas might have escaped and some should still be trapped inside the earth but the numbers are close enough to work and I think that's very likely the answer. It also suggests that the Earth has lost relatively little Argon to space, which also fits with the Atmospheric Escape article.

A 2nd way to look at this is that Argon 40 makes up 99.6% of the Argon in the atmosphere and Stellar Nucleosis likely wouldn't account for a ratio anywhere close to that (not a typical stellar link but Wikipedia says Argon 36 is the most common isotope). The decay of Potassium 40 does explain the 99.6% Argon40 ratio.

If we apply a similar estimate to Venus, with Venus atmosphere about 94 times the mass of Earth's, and we assume a similar amount of Argon-40 being produced in Venus' crust we could roughly expect 1.28%/60 or about 0.02% Argon by mass in Venus's atmosphere or perhaps, if the Earth lost a pretty high share of it's lighter crust elements after the giant impact, we might expect a bit more than that on Venus, perhaps 0.03% or 0.04% as a rough estimate. Using your number of 0.007%, that's lower than I calculate it should be, but Venus could have lost a higher share of it's Argon than Earth and it also might be slower to release trapped gas inside it's crust than Earth because it doesn't have plate tectonics, so the number for Venus looks "about right" too. It's the Potassium 40 in the crust. I'm convinced.

Interesting question. I learned something researching it.

galaxy - What part of the milky way do we see from earth?

Fraser Cain:

We’re seeing the galaxy edge on, from the inside, and so we see the
galactic disk as a band that forms a complete circle around the sky.

Which parts you can see depend on your location on Earth and the time
of year, but you can always see some part of the disk.

The galactic core of the Milky Way is located in the constellation
Sagittarius, which is located to the South of me in Canada, and only
really visible during the Summer. In really faint skies, the Milky Way
is clearly thicker and brighter in that region.

If you want to know more, watch this video from Fraser Cain which explains it in details:

https://www.youtube.com/watch?v=pdFWbEwsOmA

You can find your answer in the first minute

What would night sky look like if Earth was made of antimatter

It's kind of a strange question but I'll give it a shot.

It depend, kind of obviously, on how much of the "space dust" hits the earth. Estimates vary pretty widely on how much dust from space hits the Earth anyway, by this site, between 5 and 300 metric tons per day.

Lets start with the low end estimate, 5 metric tons (5,000 KG) per day. Antimatter will meet with and turn into energy an equal amount of matter, so 10,000 KG per day turned into energy, presumably almost all gamma-rays, but a lot of that energy would reflect off the upper atmosphere, lets estimate 40% of it reaches the earth, so 4,000 KG of gamma ray energy hits Earth from Space every day.

4,000 KG, * C^2 (300,000,000^2) = 3.6*10^20 joules of energy per day.

The sun (I'll just borrow from this link rather than calculate). 1.6x10^22, so, the heat from the matter-anti matter interactions would be 2% of sunlight. That would be enough to warm the earth measurably. A 2% increase in energy form space could warm the Earth several degrees, maybe 10 degrees, maybe more. The Earth would be measurably warmer, but that would be the least of our problems.

I think we can bring the number down a bit, cause you said no meteors, or asteroids, and even if you hadn't, larger objects like that in a matter-antimatter interaction, the contact would be so hot that most of the meteor wouldn't have a chance to evaporate but would simply be exploded away from the Earth, so we can round the number down quite a bit, like maybe a few hundred KG per day gets converted to gamma-rays and travels through the Earth's atmosphere and hits the Earth. At this stage, the total heat the Earth gets is negligible enough at those levels, but would the sky glow visibly at night? Honestly, I don't know, but maybe.

Gamma rays aren't visible, but as the rays pass through the atmosphere you might get enough visibly hot atmosphere/visible thermal radiation that you might see it. I don't know how to begin to calculate that, but it seems possible. The gamma ray effect would be similar to what would happen to the Earth if it was hit by a gamma ray burst (from a non lethal, but perhaps uncomfortably close distance). Here's a description of what that would be like. We'd lose much of our ozone layer and the chemical composition of our atmosphere would change. over time, it could end much of the life on the Earth's land. Life in the oceans could survive, but life on land would have a hard time.

That's not the only effect though. As a few hundred tons of antimatter hits the upper atmosphere, the upper atmosphere would heat up and the Earth would lose it's atmosphere due to higher temperature much faster. Over thousands, perhaps millions of years the Earth would lose much of it's atmosphere, which wouldn't be fun.

A final effect is that antimatter would change chemistry. If an Oxygen atom is hit it becomes a Nitrogen, if a Nitrogen is hit it becomes a carbon (I think, unless the gamma ray energy of the evaporation causes further photo-disintigration), but you'd see chemical changes as a result, perhaps some of them toxic and some radioactive isotopes. So, no ozone layer, gamma ray radiation and toxic chemicals forming in the upper atmosphere - not exactly sunshine and puppies.

Teeny-Tiny amounts of antimatter hitting the Earth isn't an issue, but in the amount you propose it would probably make the earth pretty inhospitable pretty fast.

What decided how the Kepler space telescope was pointed?

The prime objective of the Kepler mission was to attempt to find "Earth-like" planets using the transit technique.

To establish that you definitely have a transiting planet requires, at a minimum, that you see three regularly spaced transits.

The Kepler mission (originally) was planned for 4 years. Thus to ensure the detection of 3 transits for planets in a 1-year orbit really requires that you observe a large set of stars continuously for that period (since the transits are quite brief).

In order to do this, you need to observe a field in which neither the Sun nor the Earth get in the way during the year. This requires you to look away from the ecliptic plane.

Then, to get a large number of stars in the fixed field of view, a direction was chosen that was close, but not in the Galactic plane and viewing along a spiral arm. I believe this was done to maximise the number of stars with $V<16$ for which Kepler would supply good photometry. Closer to the plane would have given even more stars, but many would have been faint and one runs into more difficulties in terms of resolving which star is actually the variable when there is too much confusion with many sources.

black hole - Eternally collapsing objects?

With respect to "eternally collapsing", they are probably referrring to the fact that in the reference frame of an external observer, gravitational time dilation prohibits matter from ever reaching the event horizon (the "surface") of the black hole.

Denoting the radius of the event horizon $r_mathrm{S}$ (for "Schwarzschild radius"), time runs slower by a factor of $(1 - r_mathrm{S}/r)^{-1/2}$ for an observer at a distance of $r$. As $r$ approaches $r_mathrm{S}$, this factor goes to infinity, i.e. the observer will never reach $r_mathrm{S}$

However, this is only for the external observer; the falling observer would cross the horizon and reach the center in a finite time.

With respect to "not being stable", you're probably right that they are referring to Hawking radiation which presumably makes the black hole slowly "evaporate". Near $r_mathrm{S}$, pairs of virtual particles are being created and can be turned into real particles by the gravitational field. If they avoid falling into the black hole, energy is taken away from the hole, reducing its mass.

exoplanet - Are there any Stars we know don't have planets?

I am beginning to assume that our solar system is not unique and that every star has several planets.

Not quite, but indeed a study published in Nature in 2012 found that, based on our observations so far, roughly 17% of stars host Jupiter-mass planets, 52% host "Cool Neptunes" and 62% host Super-Earths. (Note that these percentages do not add up to 100%, because they are not mutually exclusive possibilities). This was particularly surprising, because half of all visible stars are believe to be in binary systems, which would make planetary systems very unstable, but some binary systems have been found to have planets too.

So indeed it seems the majority of stars have planets, but it's very unlikely that all of them do.

However, the exact answer to your question "do we know of any stars with no planets" has got to be "no", because there remains a possibility that they have planets that we simply haven't been able to detect, because of limitations in our techniques to detect them.

stellar evolution - Why do proto stars on the Hayashi track get dimmer as they contract?

The Hayashi track has an almost constant effective temperature. The pre main sequence star (not protostar) is powered by the release of gravitational potential energy caused by its contraction.

If the star is getting smaller at constant effective temperature, then of course it's luminosity ($propto R^2 T^4$) decreases.

So perhaps your question is why do PMS stars contract at constant temperature? In other words, why is the Hayashi track almost vertical in the HR diagram? The answer to this is derived in some detail on the relevant Wikipedia page and arises from the fact that low mass PMS stars are fully convective and have atmospheric opacity dominated by H- ions.

gravity - How does an absolute horizon form before the apparent horizon?

I guess you are talking about black hole formation here. If we take the Oppenheimer-Snyder model for the spherically symmetric collapse of a star, then an event horizon forms first followed later by an apparent horizon that is at, or interior to, the event horizon.

The event horizon is the surface behind which light rays will never reach an infinite distance from the black hole. The apparent horizon is the surface behind which outwardly directed light rays will move inward.

The two will coincide for a static black hole, but for a forming black hole with a growing mass, it is possible for an outwardly directed photon to be outside the apparent horizon, but inside the event horizon, because the black hole mass is inevitably larger in the future if some of its future mass is still beyond its final Schwarzschild radius. As the event horizon forms at the centre of the star and moves outwards, then it is clear that the apparent horizon, which is always interior to the event horizon, must form later.

In the very simplified Oppenheimer-Snyder model, featuring an initially uniform star comprising of pressureless "dust", the apparent horizon first forms just as the surface of the collapsing star coincides with the event horizon and the apparent horizon is therefore always coincident with the event horizon. In more realistic models the apparent horizon forms a little earlier and then moves outwards to coincide with the event horizon as the surface of the collapsing star moves inside the event horizon.

NB: Note that whilst the apparent horizon can be defined at any point in time, the "event horizon" referred to above is a theoretical ideal, since we cannot preclude that the black hole might accrete some more mass in the future!

supernova - Stellar mass limits for Neutron Star and Black Holes

A succinct summary of supernova types is given in the following image based on Heger et al. (2003):

^{Image courtesy of Wikipedia user Fulvio 314 under the Creative Commons Attribution-Share Alike 3.0 Unported license. The graph is based on the graph in Fig. 1 of the linked paper.}

The pair instability realm is upwards of ~100 solar masses, though it is metallicity-dependent (Question 3). As Figure 1 (below) shows, neutron stars form in the mass range of >9 solar masses - again, this is metallicity-dependent (Question 1a). Starting at around 25 solar masses, black holes will form (Question 1b).

I'm not aware of ways to form an astronomical black hole or neutron star not involving a Type I or Type II supernova without resorting to speculative possibilities like primordial black holes, but that doesn't make it entirely out of the question.

resource - A good website for laymen to share their discoveries?

This is a wonderful Q&A forum, but I'm wondering if anybody knows of a good website where someone can let other people interested in astronomy know about new things that they have become aware of:

For instance: Someone could inform others about what they have found out about the latest news event (ie: gravitational waves, the "megastructure" (probably not) found by Hubble, the possible new planet discovery, etc.) Another person could inform everybody about cool websites for people interested in astronomy (ie. ADS, SIMBAD, NASA's Kepler Datapage, Astrobiology On-Line Magazine, and countless others)

I just found out that there is a series of 40 full articles in "ADS" submitted from 2007 to the present titled "The HARPS search for southern extra-solar planets". If you read them in chronological order they are not only very informative, but you can see the history and evolution of the project.

I know there are numerous websites, but I am looking for one that is serious and not just an astronomy chat room.

coordinate - Protocol for establishing longitudinal meridians on other heavenly bodies

When we first observe a new heavenly body (it could be a new star, asteroid, etc., but let's say a minor planet in our own solar system), are there any procedures set in place for establishing a system of longitudinal meridians? Being that a prime meridian is an arbitrary concept that you can pick and establish anywhere, is a location decided based on a physical feature, or perhaps from the first points of data we gather when making detailed observations of a heavenly body for the first time? What about heavenly bodies with no easily discernible or non-stationary features (Gas Giants)? Also does the IAU regulate this process? I have always wondered if the USSR and the US shared common reference points for locations such as on the moon.

What does the surface of Mercury look like?

The MESSENGER probe was able to take many true-color pictures of Mercury. A full list can be found on JPL's Photojournal. It is clear that Mercury is light grey in color.

In terms of the actual surface, Mercury is very similar to the Moon. It's surface is speckled with craters, with some smaller craters inside them, as can be seen in some of the images above. The smoothness varies - note the inside of the larger crater in the first picture. It's quite smooth, save the smaller crater inside it.

See also this pdf.