Hubble - The Science Behind The Beauty
The Hubble Space Telescope is arguably the most important scientific instrument ever created. It has provided an unparalleled view into the nature of our Universe and has produced a generous back catalogue of stunning imagery. It is also responsible for many profound scientific discoveries - an achievement which frequently takes a back seat to its visual splendour.
The HST has been in orbit for nearly twenty years. As an early anniversary tribute I have selected four famous Hubble images and will attempt to explain the substance behind the style in each. I have attempted to select a broad range of cosmological phenomena to satisfy all tastes, so sit back, relax and enjoy the view!
Pillars of Creation - Eagle Nebula
The Eagle Nebula is an example of an astrophysical phenomenon known as an emission nebula. An emission nebula is a large collection of gases spread over a huge distance, heated up to such an extent that it has become ionised. A group of young stars is providing the energy to achieve this and so the Eaglue Nebula qualifies as a "H II" region. The Eagle Nebula is approximately 6,500 light-years (LY) away in the Serpens constellation. That is, it would take 6,500 years travelling at the speed of light to reach it. For comparison the Milky Way galaxy in which we inhabit, along with the nebula, is approximately 100,000 LY across. One LY is equal to approximately 10 Peta metres, or 10,000,000,000,000,000 metres.
The "Pillars of Creation", the subject of the above image, is a complex mixture of gas and dust. It was taken with the Wide Field and Planetary Camera 2 (WFPC2). The step effect occurs because the Planetary Camera in the top-right has a narrower field of view, but with more detail. The image is in fact a composition of three separate images. Each of these images was taken with light emitted by different chemical elements. Hydrogen emitted light appears in a green hue, while red is used for singly-ionised sulphur. The blue hue comes from light emitted by double-ionised oxygen atoms. Given that hydrogen is the most abundant element in the Universe, as well as this image, it is predominently green.
Pinwheel Galaxy - M101
The full-resolution version of the image above was at the time of its release in 2006 the largest and most detailed photo of a spiral galaxy ever taken. It is a composition of over 50 separate images taken over a 10 year period from Hubble as well as some exposures from ground-based telescopes. It was taken with the Advanced Camera for Surveys (ACS) and the WFPC2. The Pinwheel Galaxy - or Messier Catalogue Item #101 - is almost twice the diameter of our own galaxy, at 170,000 LY. It is thought to be the home to over one trillion separate stars.
The Pinwheel Galaxy is 25 million LY from the Solar System, meaning that the image above shows us how it looked 25 million years ago (as that it is the time it took for the light to reach Hubble!). The spiral arms of the galaxy contain many stellar nurseries, also known as H II nebulae. The Doppler shifting of light from the galaxy, coupled with measurements of its distance, allow astronomers to tweak the parameters in Hubble's Law - the law that governs the rate of expansion of the Universe.
Mars
This image was taken when Mars was only 88 million km from Earth, in one of Mars' recurring closest approaches. Since Earth travels around the Sun twice as fast as Mars, it laps the Red Planet every 26 months. The image was taken with the aforementioned WFPC2 from a series of exposures during the 36 hours of closest approach. Note that there are no stars in the image, as one might expect. The amount of light emitted from Mars is significantly higher than that of the much more distant stars. Thus, the camera exposure speed is set too high to capture any detail of the stellar background.
Many features can be seen on the planet at this depth of 21km per pixel in the full-resolution image. Large clouds coupled with the polar caps of frozen carbon dioxide surround the two poles. This provides them with a whiter shade compared to the middle latitudes. In the south a large circular crater can be seen. This is the Hellas impact basin. It has a diameter of over 1,700km and a depth of 8km. The "Opportunity" Mars Rover landed at the western end of Sinus Meridani, the horizontal path on the left of the image.
Ultra Deep Field
The Hubble Ultra Deep Field image is one of the most incredible pieces of scientific data ever obtained. It is a combination of 800 exposures taking nearly one million seconds or 11.3 days in total to produce. The image displays an extremely small area of the sky and looks back approximately 13 billion years into the past. This is almost the age of the Universe itself at 13.7 billion years. The image is estimated to contain more than ten thousand galaxies.
The region of the sky was chosen due to its relative sparsity of Milky Way stars in that direction. Only seven Milky Way based stars can be seen. The image shows a more intense, chaotic period of galactic interaction than we are familiar with. The galaxies are far more irregular and do not possess the same degree of structure or order that is present in the galaxies seen today.