Why nuclear fusion is a controlled process in stars?

Because in most stars, the pressure where nuclear burning takes place is proportional to temperature. In order for the nuclear reaction rate to increase, the temperature and hence pressure must increase. This would cause the gas to push out the layers above it, then to expand and cool.

Conversely, if the nuclear reaction rate fell, so would the temperature and pressure. The smaller pressure would allow the outer layers to move inwards, compressing the gas, heating it up and speeding up the nuclear reactions.

In this way the nuclear reaction rate (for hydrogen burning) can be kept almost constant. The small changes (increase) that occur during a star's main sequence life are driven by the increasing mean atomic mass as hydrogen turns to helium. This means you need a gradually higher temperature to maintain the same pressure.

Where the temperature and pressure are decoupled - for instance in the degenerate helium core of a low(ish) mass star near the red giant tip, where electron degeneracy pressure is independent of temperature, then runaway nuclear reactions can be initiated - the "helium flash".

immunology - What's the advantage of autocrine signalling?

In the antibody-mediated immune response, when the helper T cell gets activated by the costimulus (IL-2 and TNF-α secreted by the APC) which in turn produces IL-2, IL-2 acts in an autocrine manner. I'm just wondering why does IL-2 have to be secreted? Why doesn't it just exert an affect while it's already inside the helper T cell? What's the point of autocrine signalling?

I hope the answer isn't going to be, "Well, that's just the way it is..." because paracrine and endocrine make sense and have advantages, but autocrine just seems a bit extra.

How do astronomers detect gases that are in the atmosphere of exoplanets?

Exoplanets are planets that are located outside our solar system - whether that be orbiting a star or drifting past one. Now, the closest star to us is Alpha Centauri which is just over four light-years away. So, how can astronomers detect gases in the atmospheres of planets over this distance with any degree of accuracy (or at all)?

black hole - Explanation for the first spinning neutron star detected in the Andromeda galaxy?

A new class of Neutron stars has been found in the Andromeda galaxy (M31), This is the first time a Spinning Neutron star has ever been detected by astronomers in M31. The article states-

“pulsars” can be found in stellar couples, with the neutron star cannibalizing its neighbor. This can lead to the neutron star spinning faster and to pulses of high-energy X-rays from hot gas being funneled down magnetic fields onto the neutron star.

“It could be what we call a ‘peculiar low-mass X-ray binary pulsar’ — in which the companion star is less massive than our Sun — or alternatively an intermediate-mass binary system with a companion of about two solar masses,” said Paolo Esposito of INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Milan, Italy.

While the precise nature of the system remains unclear, the data imply that it is unusual and exotic.
http://www.astronomy.com/news/2016/04/andromedas-first-spinning-neutron-star-has-been-found

Question-
Could the companion of this Spinning Neutron Star not be a star at all but a Black Hole? This might explain the unusual spin and that all neutron stars do not consume other binary companions stars but rather a Black Hole is in the process of consuming the neutron stars.

The Black Hole can devour all of the outer energy of any active star leaving only the dense core of heavy metals that will eventually cool off entirely creating the rouge brown dwarf.
Supernovae never derived from dying neutron stars but rather from Binary star Collisions.

universe - If I were to point into the sky, how many galaxies would be in this line?

On average, you will point at one galaxy.

The argument goes as follows:

Looking at the spectrum of quasars, which are effectively point sources, and which lie at distances of the order of the size of the observable Universe, it turns out that typically, roughly one damped Lyman $alpha$ system (DLA) is detected. DLAs are huge reservoirs of neutral gas, which are early stages of galaxy formation.

At least in a handful of cases, the galaxy counterpart of the DLA has been confirmed. These observation are very challenging, since they are a faint sources at very small projected distances from the very bright quasar. Here's the setup:

So, accepting a DLA as a galaxy, I'd say that you will typically point at one galaxy.

How does gravity affect spacetime

In fact time always "moves" at the same speed to any given observer. It appears to move at different speeds to an external observer, but you cannot experience slowed time as such - except in the sense that you might observe external clocks ran faster than the watch you had your wrist.

An attempt to explain: The supposition - very strongly supported by the experimental evidence (the latest discovery of gravity waves being the jewel in the crown) is that we live in a four dimensional spacetime. An individual always experiences time as "moving" at a constant speed, but as has been remarked by others, externally gravity appears to add curvature to the "straight line" we are moving along on the time dimension and so we "gain distance" in this direction more slowly when seen externally.

Has Science observed and recorded a stars birth?

The star formation process from giant molecular cloud to unobscured protostar is thought to take about a million years.

So the answer is no.

Similarly, there are very few large scale physical processes that occur in the universe on a human timescale. Nevertheless we are sophisticated enough to understand that you do not necessarily have to see something happening to know that it has occurred and work out how it happened.

Or are you asking whether all the separate phases of the star formation process have been observed in different places? The answer to that is broadly yes. The rarest (shortest) phases is the initial collapse to a "core" that is embedded within an obscuring molecular cloud. Nevertheless, such objects can be seen at sub-mm and radio wavelengths.