What is is the interstellar medium? The interstellar medium is the space between stars and galaxies. This matter includes a collection of gas, dust and cosmic rays. It is a region of birth for stars.
By measuring a stars color we can determine its age. Blue stars are relatively short-lived compared to the redder stars. The blue stars die off over a period of a few hundred million years, but the redder stars continue to shine for billions of years. Therefore, as a star cluster ages, its color gradually shifts from blue to red.
Source: Hubblesite.org
Perhaps one of the most iconic images by Hubble, the above image of the Eagle Nebula shows large pillars of molecular gas and dust. In this region there are many young stars, the largest pillar in the image is about 3.86243exp13 kilometers or 24 trillion miles in size.
Source: Hubblesite.org
The above image shows area of young star formation.
These clouds are held together by self-gravity and by the intense pressure that builds up in between the clouds. This in effect causes these molecular clouds to collapse into a center, which draws gas towards inside the cloud. When the density is high enough the clouds end up fragmenting and collapsing into itself. From these clouds of dust and gas are born stars and planets.
The above is an area in our local Large Magellanic Cloud (LMC), where there is low-mass stars with giants beside it, known as LH 95 it is about 160,000 Ly away from Earth. The image was taken by Hubble's Advanced Camera for Surveys.
From these high detail Hubble images we are looking at regions of interstellar clouds. Self-gravity is a gravitational attracting among all parts of the same object. The sum of all these forces on every other particle is exerted towards the center. This gravity can be balanced by its own structural pressure similar to how Earth is made up from rocks, or by the outward force acting on the inward force creating a balance known as hydrostatic equilibrium. If either force is unstable the star can either expand outwards on inwards into itself. However in most clouds the internal pressure pushes outward more than inward.
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| From the above simulation we can get an idea of where the highlighted grey blob is the Large Magellanic Cloud (LMC) and the milky way largely visible on the right. |
Source: Hubblesite.org
The densest coolest interstellar clouds are called molecular clouds because they are mainly composed of hydrogen molecules. Once these molecular clouds begin to collapse the innermost becomes a star and outer parts can become planets. The innermost part of cloud is called a protostar. The surface of a protostar is thousands of times larger then the Sun and as a result is also more luminous.
Source: Hubblesite.org
LRLL 54361 a binary protostar, it demonstrates some unknowns about star formation. The protostar emits a burst of light at intervals perhaps caused by the eccentric orbit that it may exhibit.
Source: Hubblesite.org
The above is a spectacular image of blue newly formed stars in the Small Magellanic Cloud. The Small Magellanic Cloud is in the constellation of Tucana around 200,000 Ly away. This dwarf galaxy has far less stars than compared to our own galaxy.
Even though a protostar is so bright its often the case that we cannot even see it in visible light. One of the reasons is that this light is in the infrared spectrum. Also a protostar is often buried deep within molecular clouds thus making it identifiable a difficult process. The mass of a protostar determines if it will actually become a star, as it slowly collapses, the temperature in the center rises. A newly born star continues to readjust its structure until its energy that is being radiated is the same as energy being created inside of it, this hydrostatic equilibrium is crucial. 
Simulation of Milky Way being viewed from the Small Magellanic Cloud.
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| Simulation of close up a globular cluster of stars, NGC 6338.
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