This post is part of the Young Astronomers Databank Project.
In the first part of this guide I looked at the basics of spectra with particular focus on black-body spectra. In this section I will take a look at a few specific emission features and what they can tell us about an object.
So lets begin our journey …
As each element and ion produces its own set of spectral lines scientists have created a standard notation of showing which line is produced by each atom or ion.
Any produced by an atom is labelled as – (x)I where (x) is the chemical symbol of the atom involved e.g. a line produced by the atom sulphur would be labelled as SI on a spectral chart.
As mentioned above ions also produce spectral lines. In astronomy we are nearly always dealing with positive ions (those which have had one or more electrons knocked off) even with those elements, that under normal circumstances on Earth, would take on additional electrons to become negative. This is because in astronomy where emission lines are produced the energies involved are much higher than on Earth and so it is much more likely that an electron (or perhaps two or even three) is knocked off rather than attracted towards the atom. As such ions with a positive are much more common in astronomy than those with a negative charge.
An ion with a +1 charge (one electron has been stripped off) produces lines labelled as (x)II with (x) again standing for the chemical symbol of the atom involved.
An ion with a +2 charge follows the same pattern – (x)III
and so the pattern continues
Let’s now look at two specific elements:
Hydrogen produces a series of emission lines that fall within the visible section of the electromagnetic spectrum (it also produces several others that lie outside the visible range). This set of lines is called the Balmer series after the scientist who first described them Johann Jakob Balmer.
They are all produced by electron transitions to the second energy level. When displayed using a hydrogen discharge tube (a cylinder of pure hydrogen through which an electric current is passed to excite the atoms) and a spectroscope the lines can be seen like so:
As is typical in science the rule we just learned about spectral notation doesn’t apply in this case.
For historical reasons the lines of atomic hydrogen in the visible region are named as H followed by a Greek letter. Hα is the lowest energy transition – red- moving through Hβ – which is a blue-green – and then Hγ, Hδ and Hε all being shades of purple (Hδ and Hε are sometimes classed as being ultraviolet rather than visible spectral features but still follow the same naming pattern).
Hydrogen Alpha – Hα
This emission feature occurs at 6562.8 Angstroms (656.28 nm). It is produced when an electron in a hydrogen atom decays from the third energy level to the second producing a photon with 1.9 eV (where one eV is 1.66×10-23 J) of energy.
Hα is a hallmark of star forming regions. It appears pink to the human eye (its the bright red line in the image above), and is displayed as such in most professionally produced images such as this one captured by the ESO’s MPG telescope of the Large Magellanic Cloud (LMC)
Hα is also a marker for AGN activity, with most such galaxies (including quasars) display strong Hα emission.
Oxygen is the third most common element in the Milky Way making up about 10,400 parts per million in terms of mass.
As oxygen has eight electrons rather than hydrogen’s solitary one, oxygen’s spectrum is much more complex with a great deal more lines than that of hydrogen ( hydrogen has 5 spectral lines between 4000-7000 Å (roughly the visible range) compared to oxygen’s 73! Using simplistic terms because oxygen has more electrons there are more available energy levels for those electrons to jump into, that in turn means more energy level transitions are possible and so giving rise to more spectral lines, as each line represents a possible transition.
The visible spectrum of oxygen looks like:
Doubly Ionised Oxygen – OIII
Another hallmark of star forming and active regions. A blue-green line that can be exceptionally intense in certain circumstances and can completely dominate the colour of some galaxies.
In most images containing OIII data it is displayed as either green or blue such as this Hubble image of the nearby active galaxy NGC 6822 (OIII is shown as green in this particular example)
In the next post in this series I will be taking a look at a few specific absorption features and what the colour of an object in general can reveal.
Not too long ago, astronomers figured out that most large galaxies – including our own – have a supermassive black hole (an enormous object which can be billions of times the Sun’s mass) right in its core. Along with this astonishing finding, an unsettling question was raised: Which came first, galaxies or the supermassive black holes in their cores?
The cosmic chicken-and-the-egg problem
Scientists know that there is an intrinsic relationship between the galaxy and its black hole. It’s believed that black holes and galaxies grow together, in a kind of partnership. As the black hole swallows the material in the surrounding area, it becomes more and more massive, and the more massive a black hole is, the more radiation it blasts out. This powerful radiation heats up the clouds of gas in the galaxy, but it’s still unclear whether this radiation boosts or prevents the formation of new stars.
By studying the nearby galaxies, astronomers also figured out that there is an elegant relation between the mass of the supermassive black holes and the giant cloud of stars and gas in the core of their galaxies (nicknamed “bulge”): no matter how big is the galaxy, the black hole is always about 700 times more massive than the bulge. Although this suggests that galaxies and their black holes evolve together, scientists weren’t sure whether this rule was obeyed in the early universe.
Looking back in time
In order to see the first black holes, astronomers need to look as deep into space as possible. The problem is that it’s not easy to observe an ancient galaxies in details, mainly because of their high redshift. The most easily seen galaxies in higher redshifts are quasars, due to their extreme brightness. Nevertheless, separating the radiation emitted by the supermassive black hole from the glare of the quasar is a tough challenge.
To calculate the size of the Milky Way’s supermassive black hole – as we cannot actually “see” it, – astronomers needed to measure the speed of the stars orbiting in the heart of the galaxy: the faster the stars move around the black hole, the more massive it is. Measuring the size of a supermassive black hole hosted in another galaxy is an even more complex task: the scientists need to analyse the infrared light coming from the centre of the galaxy. Moreover, the data needs to be carefully filtered, in order to separate the radiation emitted by the black hole from the light coming from other objects in the galaxy.
Using powerful radio telescopes, such as the Very Large Array (VLA) in U.S. and the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile, along with image-filtering processes, astronomers were finally able to get accurate measurements of the mass of these early black holes and had a big surprise.
In the galaxies observed during the studies, the black holes were only 30 times less massive than the bulge of their host galaxy, breaking the rule we see in the modern galaxies. It appears to solve the chicken-and-the-egg mystery, indicating that black holes actually came first.
Despite this major discovery, the origins of the supermassive black holes remains one of the most hotly debated topics in astronomy. Many undergoing projects are trying to pile up evidences to either underpin or contest this theory, and better explain when and how this mysterious relation came about.
Today on the Young Astronomers we bring you an interview with the amazing astrophotographer Ken Crawford, so without further ado let’s begin:
What is an astrophotographer?
Astrophotographers come in two basic types. Solar System astrophotography are images of the Sun, Moon, and Planets. These types of images are normally taken with special video cameras to capture many pictures of the object. Because the objects are bright, the pictures are taken very quickly. You then remove any blurred images and keep the good ones
Deep sky astrophotography captures the light from very faint and distant objects like nebulae, galaxies, and star clusters. These pictures are taken with very long exposures over several hours and the combined together for the final result.
What first attracted you to astrophotography? Did anyone inspire you to take it up?
I have always been interested in astronomy and built my first telescope in 8th grade. I became interested in astrophotography because it is a technical art form. The technical part is that you assemble different imaging tools together like the telescope, mount, and camera and make them work together. Then you need to be able to learn to use the software that controls the telescope and camera and the assembling of the images. The art form is the presentation of the colors, contrast, and details in their most beautiful form possible. The amazing thing is that amateurs can produce very professional results with modest equipment, dark skies, and lots of practice.
I was inspired by some of the pioneers of astrophotography like David Malin, Rob Gendler, and Tony Hallas. I also had the support of my wife of over 34 years which is a huge plus.
What is/are your favourite object(s) to photograph?
My favourite objects are distant island universes (Galaxies) and star forming regions (Nebulae).
Does astrophotography require any special equipment, or is a standard digital camera suitable?
You can do what is called wide field low resolution work with standard DSLR cameras but the better work comes from astronomical cameras with monochrome (greyscale) CCD with color filters in front. Here is a picture of my imaging train.
A = main CCD Camera – cooled to -25c
B = Filter wheel with 10 color filters
C= Off axis pick-off mirror to send starlight to the guider camera.
D= Guider Camera
E = Rotator to rotate the complete image train to any position
Do any resources exist for beginners?
Some, online forums, books, and online telescope rentals can help. Some astronomy clubs can help out if they have Astrophotographers as members. I am president of the Advanced Imaging Conference and we have once a year seminars and classes. The online forum called Cloudy Nights has a beginner section.
Is the any advice you could pass on to any of our readers interested in starting astrophotography as a hobby?
The hobby can be expensive but you can start out very easily with just a DSLR camera and a small tracking tripod. You can capture nice images of the constellations and other large celestial objects. You can use an inexpensive webcam to capture images of the moon and bright planets. But first, join the online forums or find someone who is doing astrophotography and ask for help.
We would once again like to thank Ken for his participation with the interview and giving up his time to answer our questions!
The ESA’s Mars Express orbiter has captured this fantastic image of the Ladon Basin, specifically of this spectacular double impact crater:
The pair are named Sigli and Shambe and are believed to have been formed by a single object that broke into two larch fragments just before impacting the surface of Mars.
The shape and shallow nature of the impact crater suggest that it was formed when an asteroid or comet hits a planet at a reasonably shallow angle.
This particular pair is 16km across and shows significant fracturing of the crater floor. The pair also show signs of being partially filled with sedimentary material at some point after their formation. This implies that they may well have been lakes, as such material is only deposited under water, hinting once more of Mars’ more environmentally pleasant past.
You can read more about this image here
In around eight hours at 06:31 am, (I’m not counting, honest) the Mars Curiosity Rover will begin her descent into the Martian atmosphere and, if all of the many stages of descent and landing go perfectly, begin her mission.
The mission itself is to find out if the past – or present – environment on mars was suitable for microbial life to inhabit the soil. The mission will last as long as Curiosity does, her plutonium power source will give her enough power to be our interplanetary geologist for at least 687 days; a Martian year.
As of an hour ago Curiosity was just 142,783 km away from Mars, less than a third of the distance Earth is from the Moon. If you’d like to know plenty more snippets like this I suggest following @MSL_101 on twitter or the official NASA account, @MarsCuriosity.
I also had to share this brilliant NASA Jet Propulsion Lab video describing the challenges faced during descent. Unsurprisingly it’s described as ‘the seven minutes of terror’:
You can find a good summary of the mission here!
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