What is the difference between asteroids, comets and meteors?

The objects you are refering to are actually two different objects: asteroids and comets. Meteor and meteorite are other names for an asteroid, at a given time of its interaction with our planet. We'll get to that.

So first, what is the difference between an asteroid and a comet?

A comet is a small solar system body that display a "coma" (an atmosphere of a sort) and sometimes a tail passing close to the Sun. They are mostly made of ice and dust, as well as some small rocky particles. We distinguish two kind of comets, with short or long orbital period. The short orbital period ones originated from the Kuiper Belt, a region composed of small bodies beyond the orbit of Neptune. The long orbital period ones originated from the Oort cloud, a scattered disk of icy planetesimals and small bodies laying around our solar system.

An asteroid is a small body, composed mostly of rocks and metals. In our solar system, they can be originated from the asteroid belt, laying between Mars and Jupiter, or from the orbit of Jupiter (the Jupiter Trojans), or from actually almost everywhere in the Solar System.

An asteroid that enters the Earth's atmosphere becomes a meteor, what we also call a "shooting star".

Eventually, a meteor that was massive enough not to be completely distroyed entering the Earth's atmosphere and hitting the ground is a meteorite.

spectroscopy - Convert axis display in ds9

I have a fits file of an echellogram with the axes showing the pixel indices. However, I would like to convert my horizontal axis from pixel to wavelength given that I have a conversion formula for how much in $mu$m corresponding to 1 pixel. How should I do this so that I could see wavelength rather than pixel index directly in DS9? Thanks!

big bang theory - How do we know the universe's expansion is speeding up?

As a fellow layman, I'll give this my best shot.

The further away a galaxy is from us, the faster it is moving away
from us. But the galaxies we see exist in the distant past because it
takes a long time for their light to reach us. The closer galaxies
are, the more recent their light is, and the slower they move away
from us.

All true. Think of a single point explosion, at any given time, the object twice as far should be moving twice as fast, cause they both started in the same place at the same time, so twice as fast moves twice s far. There are, complications - with the explosion there's air resistance, with the universe well, I'll get to that.

Doesn’t this seem to indicate that the expansion of the universe is
slowing down, because the more recent light from galaxies, appears to
be moving away slower? Does this make sense?

OK, it sounds like you have an incorrect assumption there. Expansion means the far away is moving away faster and the closer is moving away slower, that's true with accelerating or decelerating expansion.

What they look for when studying expansion is measuring the precise speed and comparing that to distance. In steady expansion, you expect to see the galaxy 8 billion light years away to be apparently moving away twice as fast as the galaxy 4 billion light years away. (I say apparently cause we can only observe the relative velocity from 8 billion years in the past).

So, whether the galaxy 8 billion light years is traveling slightly more or slightly less than twice as fast away from us as the galaxy 4 billion light years away - that's what tells us about acceleration vs deceleration.

Imagine a car driving 60 MPH, and the driver puts the ever so slightest pressure on one of the pedals, but you don't know which one, and similarly the car driving 30 MPH does the same, but we can only see how fast the 60 MPH car was driving 6 minutes ago and we see how fast the car driving 30 was driving 3 minutes ago - that gives the 30 MPH car more time to apply the acceleration or deceleration. It's that time differential that gives us the information.

So, like the cars above, the galaxy 4 billion light years away has been in the accelerating expansion of space longer than the one 8 billion years ago and that's what tells us that the acceleration of expansion is happening, the nearer galaxies are moving a bit faster away from us than they would in a steady expansion or a gravitationally slowed expansion. (edited to make a bit more clear)

The newer light is moving slower. The older light moving faster. So
from this it seems that the universe was expanding faster in the past?

light doesn't move slower or faster. it red-shifts when objects are moving away from each other. The faster they move away the greater the red-shift. That's one way relative velocity can be measured.

biochemistry - What are the variables that control/influence the color of oranges(Citrus sinensis)?

I hear that Oranges cultivated in tropical areas of the world tend to be greener when ripe, is that correct?
Even the same type of Orange differs in color if cultivated in California or Florida. I hear that's because of the climate (colder nights == oranger oranges)

But, at the same time, oranges tucked in between the tree leaves tend to be greener, for they need more chlorophyll to make the most use of less access to sun rays.

Could someone correct my assumptions? Also, if possible, list the pigments and processes involved?

cosmology - Evolution of the Hubble parameter

The solution to the Friedmann equation in a flat universe is
$$H^2 = frac{8pi G}{3}rho + frac{Lambda}{3},$$
where $rho$ is the matter density (including dark matter) and $Lambda$ is the cosmological constant.

As the universe expands, $rho$ of course decreases, but $Lambda$ remains constant.

Thus the Hubble "constant" actually decreases from its current value $H_0$ and asymptotically tends towards $ H = sqrt{Lambda/3}$ as time tends towards infinity.

As $Lambda = 3H_0^{2} Omega_Lambda$, and measurements suggest that $Omega_{Lambda} simeq 2/3$, then $Lambda simeq 2H_0^2$, and the Hubble parameter will therefore decrease to approximately $sqrt{2/3}$ of its present value if the cosmological constant stays constant.

Of course if $Lambda = Lambda(t)$, (ie not the basic $Lambda$-CDM model) then the behaviour will be different.

EDIT: Another useful form of the solution (for the case of a constant vacuum energy density) is

$$H^2 = H_0^2 left( frac{Omega_r}{a^4} + frac{Omega_M}{a^3} + frac{Omega_k}{a^2} + Omega_{Lambda}right),$$
where $H_0$ is the Hubble parameter now, $a(t)$ is the scale factor of the universe, $Omega_r$ is the current (i.e. $a=1$) ratio of the radiation density to the critical density and $Omega_M$, $Omega_k$ and $Omega_{Lambda}$ are the equivalent densities for the matter (baryonic and dark), curvature and (constant) vacuum energy densities.

As $a$ increases you can see that all three of the leading terms get smaller and the Hubble parameter decreases at all times. When $a$ is very large, $H$ approaches $sqrt{Omega_{Lambda}} H_0$ as before.

expansion - What exactly is meant by "expanding universe"?

When we say the Universe is expanding, we really mean that space is expanding. The Universe could be infinite in size, but there continues to be more and more space between objects. Essentially, objects are moving away from each other because more and more space is being created between them.

In the early Universe, matter was much closer together than it is now; this density caused extreme temperatures and no hadronic matter could form. The first expansion is thought to have occurred because of how hot and energetic everything was, and spacetime itself was expanded in an event we call the Big Bang.

As objects spread out from each other during the inflationary epoch, the Universe began to cool down. Now, dark energy is the main culprit for its accelerating expansion.

To answer your question, yes, dark energy is making all particles move away from each other, although farther objects move away from each other at a faster rate. Why is that?

Well, the only thing holding all atoms, stars, planetary systems and galaxies together are the four fundamental interactions — the strong force, weak force, electromagnetic force, and gravity. In space, the main attracting interaction is gravity. As such, dark energy will cause farther objects (which are less affected by our gravity) to move away at a faster and faster rate.

Think of it like you are holding a dog with a leash, and the ground begins to spread out from below you. You and your dog are being moved away in opposite directions, but you both stay close together because of the strong leash.

Now, your mention of atoms being torn apart is actually the basis of the Big Rip hypothesis. It states that dark energy will become more and more abundant until even closeby objects separate from one another. First, galaxies will be disbanded as the stars move away from each other in all directions.

Much later, the gravity of planetary systems won't be enough to hold them together. Later still, planets and stars will be disbanded into their molecules. This will eventually continue as the amount of dark energy increases, until not even atoms can be held together by the electromagnetic force, and the subatomic particles will be moved apart.

By the time the expansion of the Universe exceeds lightspeed at the subatomic scale, no particle will ever be able to interact with one another. The Universe would dissolve into countless lonely particles that won't be able to do anything. Now, the Big Rip hypothesis is only one of the three most known fates of the Universe. Currently, the most accepted theory is the heat death of the Universe.

Life without DNA? - Biology

To follow up what mbq said, there have been a number of "origin of life" studies which suggest that RNA was a precursor to DNA, the so-called "RNA world" (1). Since RNA can carry out both roles which DNA and proteins perform today. Further speculations suggest things like a Peptide-Nucleic Acids "PNA" may have preceded RNA and so on.

Catalytic molecules and genetic molecules are generally required to have different features. For example, catalytic molecules should be able to fold and have many building blocks (for catalytic action), whereas genetic molecules should not fold (for template synthesis) and have few building blocks (for high copy fidelity). This puts a lot of demands on one molecule. Also, catalytic biopolymers can (potentially) catalyse their own destruction.

RNA seems to be able to balance these demands, but then the difficulty is in making RNA prebiotically - so far his has not been achieved. This has lead to interest in "metabolism first" models where early life has no genetic biopolymer and somehow gives rise to genetic inheritance. However, so far this seems to have been little explored and largely unsuccessful (2).

edit

I just saw this popular article in New Scientist which also discusses TNA (Threose nucleic acid) and gives some background reading for PNA, GNA (Glycol nucleic acid) and ANA (amyloid nucleic acid).

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(1) Gilbert, W., 1986, Nature, 319, 618 "Origin of life: The RNA world"

(2) Copley et al., 2007, Bioorg Chem, 35, 430 "The origin of the RNA world: co-evolution of genes and metabolism."