In the 1800s astronomers began to light from the Sun through a diffraction grating, which allowed them to view the Sun’s spectral lines. Thus began an area of astronomy known as spectroscopy. As the technology advanced enough to look at the spectra of stars, we were finally able to categorize stars by not just their temperatures, but by the elements contained in their atmospheres. We could also use the shift of these spectra due to the Doppler effect to determine a star’s motion. With modern telescopes we can do the same for galaxies using a method known as long-slit spectroscopy.

Spectral line along a galaxy. Credit: Wikipedia

The idea of long-slit spectroscopy is to only observe the spectrum of an object along a narrow line. In the case of galaxies, this is typically along its long edge when a galaxy is viewed from mostly edge on. In this way we can look at the spectrum all along a galaxy. Because spectral lines are shifted toward the red or blue due to the motion of the source, the rotation of a galaxy gives a spectral line a shift. From this we can determine the motion of stars in the galaxy. This allows us to study things such as dark matter, which can affect the motion of stars.

Long-slit spectroscopy also allows us to study things such as the evolution of galaxies over time, and how the composition of stars can vary based upon their distance from the galactic core. Given how faint galaxies are compared to many stars, it’s actually a pretty amazing type of astronomy.

The post Along the Line appeared first on One Universe at a Time.

The Pillars of Creation is perhaps the most famous nebula in the night sky. One of the controversies regarding the pillars is whether they still exist. The nebula is about 7000 light years away from us, so what we see is the pillars as they were 7000 years ago. Observations of the Spitzer infrared telescope found evidence of a supernova shock wave, that by some calculations will destroy the pillars in about 6000 years. That would mean the pillars were destroyed about a thousand years ago. But we now know that isn’t the case, thanks to a bit of 3D imaging.

An infrared view of the pillars showing hot young stars in the nebula. Credit: NASA/ESA

The pillars were observed with an integral field spectrograph known as MUSE. This device allows us to measure the spectrum of an image at multiple points at the same time. From this we can determine not only where a particular pillar is in our field of view, but also how far away it is relative to other pillars. From this we can determine which parts of the pillars are behind the young O and B stars of the nebula, and which are in front. Using this data, a team of astronomers calculated the rate at which these bright stars are causing the pillars to evaporate. They found the pillars are deteriorating at a rate of about 70 solar masses per million years. Since the mass of the pillars is about 200 solar masses, it will be about 3 million years before they are completely destroyed.

That’s a short time on cosmic scales, but it also means that the Pillars of Creation are indeed still there, and will be for quite some time.

Paper: A. F. McLeod, et al. The Pillars of Creation revisited with MUSE: gas kinematics and high-mass stellar feedback traced by optical spectroscopy. MNRAS 450 (1): 1057-1076 (2015).

The post Depth of Field appeared first on One Universe at a Time.

One of the properties of atoms and molecules is that they interact with light in an interesting way. If you heat up atoms or molecules in a gas, they will give off light. But they only give off light at specific wavelengths (colors).

The particular colors they emit depends on the type of atom or molecule it is. So one type of atom might give off red, orange and blue, while another might give off yellow and green. We can look at the full range of colors a particular atom emits, which is known as an emission spectrum. You can see an example of such a spectrum in the figure below.

Credit: Moussas Xenofon and Strikis Iakovos

If you put a cool gas in front of a light source you get what’s known as an absorption spectrum. Basically this looks like a full rainbow of colors with dark lines at particular wavelengths where the gas absorbs that color. The colors a cool gas absorbs are the same colors a hot gas emits.

Each type of atom or molecule has a specific emission spectrum. It’s kind of a chemical fingerprint that allows us to identify the type of atom or molecule makes it. That is particularly useful in astronomy, because when we observe a particular pattern in starlight we know a particular atom or molecule is in the star’s atmosphere. The spectra are also affected by things like temperature and magnetic fields, so we can learn lots of things about a star by looking at its spectra. The closest star to us is our Sun, so we can make very detailed observations of its spectra. One of the tales these spectra tell is a bit of a mystery.

The Sun’s atmosphere can be divided into the photosphere, chromosphere, and corona. The photosphere is the layer where light is emitted from the sun. Within the photosphere we see an absorption spectrum This makes sense because we would expect the atmosphere of the Sun to get cooler at higher levels. The absorption spectrum means the upper layer of the photosphere is cooler than the lower layer.

One would expect the chromosphere to be cooler still, but within the chromosphere we see emission spectra. This means the chromosphere is actually hotter than the photosphere. At its lowest region the chromosphere is about 4,500 K, but at its upper region it is about 25,000 K. The corona is very diffuse, but its temperature is even higher, on the order of a few million Kelvin.

So what’s going on? Why is the upper region of the Sun’s atmosphere so much hotter than the lower region? We aren’t entirely sure, but we have a few ideas. A driving factor is that the solar atmosphere is a plasma. This means it interacts strongly with magnetic fields. The magnetic field lines of the Sun can be twisted by the motion of the plasma up to a point, but eventually snaps back into place, releasing energy. (I wrote about this in a post earlier this week). Another factor is that the chromosphere is very active with solar flares, prominences, etc., and the energy from these tends to heat the chromosphere.

What we aren’t entirely sure of is what mechanism causes the corona to become so extraordinarily hot. Plasma physics is complex, and we’re still figuring it out.

The post Light Me Up appeared first on One Universe at a Time.

Spectroscopy is one of the most useful tools of modern astronomy.  With it we can identify the atomic and molecular composition of celestial objects, we can measure the relative motions of stars, and we can observe the expansion of the cosmos.  While early spectroscopes used prisms to break light into a spectrum, modern telescopic spectroscopes generally use a device known as a diffraction grating.

A prism is able to separate light into colors due to the fact that different colors bend at different angles when passing through a material.  You might remember this effect as the cause of chromatic aberration in telescopic lenses.  A diffraction grating uses the the wavelength of the light itself to create a spectrum.

Red laser light passing through a diffraction grating. Credit: Wikipedia

Since light has properties of a wave, it spreads out in all directions from a source, just as ripples caused by dropping a stone in a pond.  Just as water waves can interfere and overlap, so can light waves.  The resulting effect that we see is due to the sum of all the light waves.  A diffraction grating is a fine grating of reflective surfaces, kind of like a reflective picket fence (but tiny).  When light strikes the diffraction grating, it is reflected from each different grating.  The light from each of those reflections spreads out in all directions, and they all interfere and overlap each other.  Because of the spacing of these gratings, different wavelengths of light are favored at different angles.  At each different angle of reflection we see a different wavelength.

There is a specific mathematical relation between the angle of reflection and the wavelength, so with a good diffraction grating we can get a very precise measure of the wavelength.  This makes it possible to make precision measurements of starlight and compare them to measurements done in the lab.  From this we can determine the properties of stars and galaxies.

The post Rainbow Star appeared first on One Universe at a Time.