Today is actually a very important day for us astronomers and astrophysicsts: it's the 25th birthday of the Hubble Space Telescope, which is one of the most awesome things humanity has ever built. 25 years after its launch, 30 years after its construction, and 38 years after its original proposal, it is still one of the most powerful and versatile telescopes we have; even new, state of the art ground-based observatories like VLT and Keck have a hard time matching its performance.
Infrared image of the same star cluster taken with the Hubble telescope, and with the ground-based Very Large Telescope, one of the best observatories ever built. Note how Hubble's resolution is better, even though the telescope itself is considerably smaller and more simple than VLT.
In this blog i'll illustrate a few basic astrophysical notions, along with some pretty pictures taken by Hubble. Most of the pictures are taken either from Hubble's webiste (hubblesite.org), or from the commemorative slides found on Nasa's website.
HST has gone through a messed up mirror that for a bit massively decreased its performance, 4 servicing missions with Space Shuttles that changed every instrument on board, a couple of broken gyroscopes that eventually got fixed, and a few failures on most instruments mounted on it. Despite this, the telescope is still able to operate at near 100% efficiency (which is pretty much unheard of for such an old satellite), and over its lifespan the data gathered has produced hundreds of scientific papers, a number unmatched by any other instrument minus the ground telescopes of the ESO observatory.
This is a picture of the galaxy M100 right after Hubble's launch, and after the first maintainance mission. Hubble's main mirror is slightly deformed - the curvature is off by 0,0022 millimeters, resulting in massive distortions. The first maintainance mission added correptive optics to fix this flaw, resulting in much better resolution.
One of the most important concepts in astrophysics is that, because the speed of light is finite, looking at objects farther away means we are actually looking back through time: light takes about 8 minutes to travel from the Sun to earth, about 4 years from the closest star to the Sun, 2 and a half million years from the closest massive galaxy. The farther away objects - which are also the faintest, whose light has a harder time reaching us - are also the oldest. The ability to see faint objects then becomes extremely important when one is trying to study the evolution of the universe.
The EM Spectrum and the atmosphere
Currently, the only way for astrophysicists to study any celestial body is to analyze the light it emits. However, simply being on Earth strongly limits the light we actually can see: our atmosphere only allows through a tiny portion of the entire spectrum, called the optical or visible range, as well as some radio waves. Anything more energetic, like ultraviolet light, x and gamma rays, or less energetic, like infrared, microwaves and some radio waves, are absorbed by our atmosphere; the only way to see a source emitting X raysthen is to have a satellite outside of the atmosphere, dedicated to that particular interval of the electromagnetic spectrum.
The atmosphere lets visibile and radio light pass, but any other kind of radiation is blocked, meaning we need satellites to see Ultraviolet or Infrared light for example. This is one of the reasons why space observatories are so useful.
The atmosphere is problematic for a second reason, called atmospheric seeing. When we observe the stars from the ground, they appear to twinkle; this isn't caused by the stars themselves, but by any kind of turbolence present in the air, which distorts the original image. This distortion strongly limits the resolution of any telescopes on the ground; it can be corrected to a certain extent, but it's a very expensive procedure that still doesn't produce perfectly still images. This is the second reason why putting a telescope like Hubble in space makes perfect sense, even though it is designed to see light from the (near) ultraviolet to the near infrared, which would mostly be accessible from the ground.
This is a scientific image of two stars orbiting each other, before and after removing the atmosphere's turbolence; the difference is massive. Hubble, being in space, doesn't need this correction.
The Solar System
Hubble allows for very precise imaging and studies of planets in the Solar System; because it's so accurate, only a spacecraft actually visiting a planet can take higher quality pictures. The main advantage of using Hubble instead of a space probe to study a planet is that it's possible to look at each planet periodically, to study its changes and gain a unique insight in the object's geology or metereology. The following pictures show respectively the evolution of Jupiter's Great Red Spot, a massive storm that has lasted at least 150 years and covers an area comparable to the size of the Earth, and polar auroras on Jupiter and Saturn much like those that happen on Earth, showing that both these planets possess strong magnetic fields. The interaction of these magnetic fields with any charged particle present in space is what causes auroras, just like on Earth.
Stars and Nebulae
Some of the most known and prettiest images taken by Hubble are those of nebulae, like the Crab and Cat's Eye nebulae or the gas clouds known as the Pillars of Creation. The term nebula is actually really generic; until about the 1900's it meant any observable object that didn't have a spherical shape, like a star or a planet, but instead looked kind of like a cloud (nebula means cloud in latin). Since then we have learnt that those fuzzy patches of diffused light can be broadly classified in several types:
1) Star formation regions. These are massive clouds of hydrogen gas; because of gravitational instabilities, sometimes this hydrogen collapses on itself, heating up to the point of igniting nuclear reactions within it and forming a new, bright, hot blue star. This happens over and over in a star formation region; indeed, identifying these clouds tells an astronomer wether a certain galaxy, or a region of a galaxy, is actively forming new stars or is not.
One of the most known star forming gas clouds in the Milky Way Galaxy, called the Pillars of Creation. The first two images are in visible light, and show the gas clouds. The third one is taken in infrared light, which isn't observed by the clouds and allows to see the stars behind them.
2) Planetary nebulae. These clouds are created as a star evolves. During its lifetime, it will expel most of its envelope, until only a small core of the original star (called a white dwarf) remains. This is also how our own Sun eventually will evolve, about 6 billion years from now.
The Cat's Eye nebula is what remains of a star very simliar to our Sun at the end of its evolution: the envelope has been expelled, and all that remains is a small white dwarf star, the white dot at the center.
The Carina nebula contains the envelopes of several stars, including some between 10 and 100 times bigger than our Sun. Instead of turning into a white dwarf, they eventually will explode in massive supernovas, leaving behind a neutron star or black hole.
3) Supernova remnant. Supernova remnants are also created by a star's evolution, but their history is much more traumatic. When a very massive star reaches the end of its life, rather than "peacefully" losing its envelope and leaving a tiny remnant behind, it explodes in a massive explosion, called a supernova, possibly leaving a smallish, but very compact object, either a black hole or neutron star (basically a massive clump of neutrons). The matter that doesnt form the central object is ejected at massive velocities and forms a cloud-like structure around the original place where the star was.
The Crab nebula is what remains of the explosion that actually gave supernovas their name. The light of the explosion reached Earth in 1054, and its sudden appearance in the sky was recorded by astronomers all over the world, who thought they were seeing an incredibly bright new star, hence the name supernova (super new in latin).
4) Dark Nebula. This kind of nebula isn't made of simple elements like the others, but of tiny grains of more complex molecules, which astrophysicists simply refer to as "dust". Dust has the particular property of absorbing most visible light, thus obscuring any object behind it at these wavelenghts. In order to penetrate a dust ring it's necessary to observe at some other wavelength.
Hubble has also given significant contributions to exoplanet research - looking for planets around stars far away from ours. This is usually done in two ways: either one blocks out the star's light with some filter, and tries to catch a glimmer of light reflected by any eventual planets that might be present, or tries to catch the "footprint" of the planet as it orbits in front of the star and absorbs part of its light.
In this image, the central star is blocked out; the tiny dot the arrow points to is a planet reflecting the star's light. As the planet has orbited the star over the years, its position has changed slightly
Galaxies come roughly in two groups - ellipticals and spirals. As the name suggests, spirals are your typical, pretty galaxy that seems to have a central luminous "bulge", and several spiral arms that envelope the bulge. Ellipticals on the other hand are usually a bit less exciting - they appear as spherical or near spherical clumps of stars, almost like a spiral's central bulge without any arms.
These two kinds of galaxies are massively different from each other. Spirals tend to be filled with gas and star forming regions, leading to very young stellar populations that emit mostly blue and ultraviolet light. Ellipticals on the other hand show very little trace of gas, having already converted most of it in stars, or lost it through some other process. Their stars are very old, which means they mostly emit red and infrared light. This is why the are classified as "red and dead" galaxies. Spectroscopy shows that while a spiral galaxy rotates in a coherent way around its central axis, ellipticals don't have any notable "group" rotation.
The Whirlpool Galaxy, whose technical name is M51, is a prototypical spyral galaxy. It is interacting with a "companion" galaxy, and eventually the two will merge and for a single galaxy.
PGC-6240, known as the Rose Galaxy, and also the "twins" NGC 4038/NGC 4039, aka the antenane galaxies are also a spiral galaxies undergoing a merger.
M104, called the Sombrero galaxy, is an elliptical galaxy, but it is surrounded by a ring of dust which absorbs visible light.
M87 is a massive elliptical galaxy; there is no trace of the massive gas clouds present in spiral galaxies.
Cosmology is the study of the Universe on large scales - rather than focusing on individual objects like stars, clouds or galaxies, cosmologists study the structure of the entire Universe, its "shape", the way matter is distributed in it, its age, how it's expanding, and so on. In order to study exactly how matter is distributed in the universe, of course, we need to know the distance of the objects we see, which isn't nearly as simple as it sounds. Imagine you are seeing a faint dot of light in the sky; how can you tell wether it's a very close, not weak source of light, or really far, but powerful enough to reach us? For most astrophysical objects, it is impossible.
A special exception to this rule is a category of objects called "standard candles". These are objects whose inherent luminosity is known or easily computed, regardless of distance. Once we identify several of these objects it's then very easy to tell which ones are closer or farther away from us: the brightest will be closer, and viceversa. The tricky thing is that this process applies to very, very few phenomena and objects.
The most famous of these are particular stars called Cepheids. Cepheids are massive, pulsing stars whose luminosity varies over very regular periods of a few days or weeks. The length of the period is related to the star's absolute luminosity: the longer the period, the brighter the star. In fact, it's thanks to this kind of stars that astronomer Edwin Hubble in the 1920's first discovered that the universe was expanding: far away galaxies are also running away from us faster, following a very simple linear law: v=H * d, where v is the velocity, H is a constant, d is the distance. The Hubble telescope among its other great contributions to cosmology has helped in making more accurate measurements of the distance of Cepheid stars as well as other standard candles, improving the precision with which the constant H is known.
Hubble has also been able to take pictures of the farthest galaxies ever seen, in an observation survey called the Hubble Deep Field (and later, Ultra and Extreme Deep Fields). The telescope pointed at what seemed to be a mostly empty area of space, watching the same tiny area of about one thirteenth-million of the total area of the sky, over several months, searching for the faintest objects we had ever seen. The result was discovering roughly 10000 galaxies, some of which had formed as little as 450 million years after the Big Bang
Part of the Hubble Ultra Deep Field. The two objects that seem to form a "cross" of light are very, very faint stars, while all the others are galaxies.
This survey among others has helped our understanding of the so-called Cosmic Web. Matter tends to clump up, forming stars, which then form galaxies, which tend to clump up in galaxy clusters of thousands of galaxies, leaving immense voids between them. Clusters in turn also clump up in massive structures called superclusters,4. If you could zoom out and see how superclusters are distributed, you would see something that looks like a web, or a sponge: superclusters are tendrils of relatively high mass density, with massive voids in between them:
The Cosmic Web. The red and yellow areas are high density superclusters, while the darker ones are the voids between them.
I hope this was a good and educational read for anyone that made it all the way to the end! If you have any questions on anything, or if you would like to know more details, feel free to let me know!