star formation - How can we explain high redshift numbers?

I just finished an introductory astrophysics course$^1$ and I have a lingering question that I can't seem to resolve.

We learned that for the first few hundred million years, the universe was pretty boring and not much interesting$^2$ happened. We also learned, correctly or not, that the first stars started forming somewhere between 500 million and 600 million years after the Big Bang.

We also learned how to use redshift values to calculate age, and we talked about how the highest $z$-number we've discovered is some galaxy (GN-z11) at roughly $z=11$.

Using this calculator, we calcultaed that this galaxy apparently formed about 410 million years after the Big Bang.

So, this galaxy seems to be older than when astrophysicists think star formation happened. How can this possibly be? Clearly one of my assumptions is wrong, so is it:

• Stars actually started forming before 500 million years post-Big Bang.


• Using the UCLA calculator to calculate age is technically incorrect.


• A galaxy doesn't need stars to be considered a galaxy.


• Some other assumption I made is wrong which makes this post invalid.

As a follow-up question, what happens if we keep finding galaxies at higher $z$-numbers? At what point do we need to reconsider our theory about what happened in the "early" universe?

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$^1$I'm not an astrophysics major, so forgive any blatant falsehoods in this post.

$^2$On a macro-scale, at least.

How to determine the mass of a runaway star?

OK, I'll assume you are talking about this star or some other high-mass "runaway star".

Most massive stars are born in rich clusters and are often part of multiple system. Is not unusual for such stars to get ejected from these regions are high velocities, either as a result of dynamical interactions in multiple systems or when they are companions to stars that explode as supernovae.

The steps to estimate the mass are: (i) Obtain very detailed optical and UV spectroscopy. From this you estimate the spectral type of the star, it's effective temperature and its (current) mass-loss rate. (ii) You combine this with a known distance (the star in the link was in the Large Magellanic Cloud, so the distance was known fairly accurately) to estimate a luminosity. (iii) You compare the luminosity, temperature and mass-loss rate with the predictions of a specialised high-mass stellar evolutionary model, which includes mass-loss and rotation. The comparison yields an estimate of the mass and age of the star.

The details are found in Evans et al. (2010). It is probably fair to say that the estimate of $90 M_{odot}$, is uncertain by a few tens of solar masses.

Selecting a Telescope for Viewing Planets

Is it possible to view Saturn in little yellowish and Mars in little
reddish using following telescopes?

It is definitely possible to observe the rings of Saturn with telescopes this size. Even the Cassini division should sometimes appear visible, if the instruments are well collimated and seeing is not too bad. In terms of color, Saturn is just a boring buttery-yellow even in bigger scopes, so I wouldn't worry about that.

But Saturn is getting lower in the sky these days. If you hurry up and get the scope quickly, you may catch it for a few weeks at sunset, low in the western sky. After this, you'll have to wait until next year.

Mars is a different animal. Most of the time, all you'll see is a bright brick-red round dot, even in a bigger scope than these ones. But every couple years Mars is at opposition, when it's closest to Earth. We just had one a few months ago. Then you can see some of the big features, such as Syrtis Major, or the polar ice caps, or Hellas Basin full of frost or fog, like a big, bright white area.

However, that's only doable briefly around oppositions. The scope must be in perfect collimation, and seeing must cooperate.

http://en.wikipedia.org/wiki/Astronomical_seeing

If everything is at maximum parameters, I'm sure you could see Syrtis Major in a scope this size. During the last opposition, I've seen all of the above features, plus more (Utopia Planitia, Sinus Sabaeus, etc), in as low as 150 mm of aperture, in a scope with great optics, perfectly collimated, during nights with excellent seeing.

Anyway, for Mars you'll have to wait until the next opposition, in May 2016.

Later this year, in December, Jupiter will start rising in the East, and you could use your scope to watch it - an aperture like this is enough to see the 4 big moons and at least 2 equatorial belts. It will be high in the sky at a comfortable time in the evening early next year.

Until then, you can always observe the Moon, two weeks out of every four.

Also, the planets and the Moon are not the only things accessible with this aperture. Most of the Messier objects are visible in a 100 ... 150 mm scope, even in suburban areas. M13 is spectacular at any aperture above 100 mm. The Great Orion Nebula is awesome even with binoculars. The Pleiades are great too. Most of these deep space objects require low magnification for the best view, but every case is a bit different.

Plenty of double stars out there, too: Mizar, Albireo, even Polaris. All visible in small apertures.

I am going to buy one of them. Which one is worth more for the money
with the price difference?

The instruments are about the same. In theory, the bigger one has a bit more resolving power and a bit larger collecting area, so theoretically it should be slightly better.

In practice, with mass-produced instruments like these, it usually depends on the build quality, which can vary.

The smaller instrument is an f/8. The longer focal ratio means less aberrations; it also makes it easier for cheap eyepieces to function well, whereas at f/6 ... f/5 a cheap eyepiece may start to exhibit aberrations of its own (independent of telescope aberrations).

Also, an f/8 is easier to collimate than an f/6.

Overall, I would look at it as a matter of price. If you can easily afford the bigger one, get it. Otherwise, the smaller instrument might be a bit easier to maintain, is less demanding in terms of optics, and it should perform pretty close to the other one - all else being equal.

But since you're focused on planetary observations, remember this:

It is far more important to learn to correctly collimate your telescope, and develop it into a routine whereby you do a quick collimation check every time before you observe - it only takes a couple minutes. For planetary observations, the smaller telescope, in perfect collimation, will perform far better than the larger one, uncollimated. Heck, the little scope, perfectly collimated, will perform better on planets than a MUCH larger telescope, uncollimated - that's how important collimation is.

Improper collimation, or lack thereof, is one of the major factors for lackluster performance for a majority of amateur telescopes (along with poor quality optics - but there's nothing you can do about that, whereas collimation can be improved).

Search this forum, or just google, the term collimation, and read the numerous documents you'll find. Or start here:

http://www.cloudynights.com/documents/primer.pdf

Or here:

How can I collimate a dobsonian telescope with a laser collimator?

The owner's manual should also provide some recommendations regarding collimation (I hope).

gravity - Does a self-gravitating gas necessarily develop turbulence?

Though we may still doubt the exact driving mechanism of turbulence in each particular case (ISM, MC, circum-BH/-stellar discs, atmospheres...), can I say that in a sphere of dark matter particles no turbulence develops because they are collisionless? And on the contrary, a sphere of self-gravitating fermions inevitably develops turbulence (in this or another way) because the particles experience collisions? (Even in case of magnetic turbulence, the dissipation of the currents is due to collisions).

history - When was the term "orbit" (in the modern sense) first used and by whom?

The word orbit could refer to three latin words : orbis, which means "ring", orbitus, which describes et circular shape, and orbita, which describes the track of a wheel.

As you can see, the meanings of this word are quite old. It is therefore hard to tell the first time it was used to describe a celestial body's round trajectory. But the word itself comes from latin.

the sun - Is a planetary system star's referred to as their sun?

The technical name (in English) for the Sun is not Sol, which is just Latin for sun. The technical name for the Sun is the Sun. Another body in the sky has a similarly boring name, the Moon. There's one more boringly named object: in the Solar System: The Earth. Note the use of "the" (a definite article) and the use of capitalization to indicate a specific object.

The problem with Sun, Moon, and Earth is that we have been using these names (or their predecessors) for thousands of years. For example, sun, sol, ἥλιος (helios), and a bunch of other names for that very bright object in the sky whose presence distinguishes day from night all derive from the same proto-Indo-European word sóh₂wl̥.

As an end note, from http://curious.astro.cornell.edu/about-us/159-our-solar-system/the-sun/the-solar-system/4-what-are-the-names-of-the-earth-moon-sun-and-solar-system-beginner,

You may read or hear people using Luna for the Moon, or Terra or Gaia for the Earth, or Sol for the Sun, but in English-speaking countries, these are poetic terms, often seen in science fiction stories, but not used by astronomers in scientific writing. In some countries where Romance languages are spoken, these terms are the official names.

It's also interesting to note that most astronomers do not call our galaxy the Milky Way in technical writing--they call it the Galaxy.

Can a probe orbit Pluto given Charon's gravity?

Avoiding hard math, which I'm not very good at, the simple answer is yes, provided there's enough distance. Pluto/Charon have 4 moons orbiting them a bit further away, all in relatively stable orbits. Source

Here's distances to scale. - same source.

Because the ratio of gravitational field strength to size is exponential (Power of 1.5), the apparent closeness of Pluto and Charon to the 4 small moons looks unusual, but it's perfectly acceptable for objects of that size.

Lagrange point stability requires a mass ratio of about 26 to 1. (.0385 to 1 per source). The Earth has satellites in unstable Lagrange points, so it's certainly doable, it's just not technically "stable".

The hardest part about getting a satellite to orbit Pluto is that Pluto's sufficiently small that any ship that approaches it would need to slow down significantly on it's own to get captured into a Pluto Orbit. That's why New Horizon was a flyby, not an orbit.