X-rays from stars

Tomorrow I'm going to a small conference at the University of Warwick to talk about X-ray emission from stars and our Sun. It may surprise many of you to learn that our Sun is a relatively bright source of X-rays, and it certainly surprised most astronomers when this was discovered in the 1940s!

Why is this such a surprise? Well, the surface temperature of the Sun is a relatively modest 6000 C, which may seem like a high temperature to you and I, but its certainly not hot enough to emit X-rays, which require temperatures of millions of degrees. And yet when astronomers launched the first rockets above our atmosphere in the 1940s they found that the Sun was a bright X-ray source!

X-ray image of our Sun taken from the Solar Dynamics Observatory
(Credit: NASA/SDO)

As it turns out these X-rays don't actually come from the surface of the Sun but from a region above the surface called the corona. You can see this in the X-ray image above, which shows the corona reaching above the surface of the Sun. The corona is a hot, low density plasma of gas, which reaches temperatures of millions of degrees, making it very bright in X-rays but almost completely invisible in the optical part of the spectrum.

So why is the corona so hot if the surface of the Sun is much cooler? Well this is actually an open question that many astronomers are still trying to answer, and its a question I've tried to answer by studying other stars. Our current best guess is that the corona is heated by the release of magnetic energy generated deep within the Sun in a type of dynamo. Unfortunately we can't see within the Sun to work out how and why this is happening, which is why this is still an unanswered question. Hopefully tomorrow's meeting will shed some light on this topic!

Milky Way Astrophysics from Wide Field Surveys - Part I

The entrance to the Royal Astronomical Society's
headquarters at Burlington House in London
(Credit: Wikimedia Commons)

This week I'm at a conference in London at the headquarters of the Royal Astronomical Society where we're discussing scientific results from recent wide field surveys of the Milky Way. Wide field surveys is just another name for surveys that cover a large area of space, and there are many surveys these days that fit that category, including a few that I work on.

Because these surveys cover such a large area of space they allow many different types of astronomical objects to be studied, from young stars to old stars, individual objects to the entire galaxy. So a conference like this is a great opportunity to stay in touch with a wide array of scientific results.

Today's talks have mostly been given by the leaders of the surveys, who have been telling us about their surveys, how we can get the data from the surveys, and highlighting some of the scientific results. This is a good opportunity to learn about new survey data and to think about how this data might be useful to solve some of the problems I'm trying to address.

The Milky Way - home of many many surveys! (Credit: ESO)

I've been really impressed with the surveys presented today. They've covered (almost) every part of the electromagnetic spectrum, from radio waves through the infrared and up to the optical part of the spectrum, and they've offered up a huge range of possibilities for future work. They also have some amazing names, including such gems as e-MERLIN and UWISH - astronomers really love acronyms!

The highlight of the day for me was probably a presentation about a sub-mm survey called ATLASGAL. The sub-mm part of the electromagnetic spectrum is between the infrared and the microwave parts of the spectrum. One of the advantages of observing in this part of the electromagnetic spectrum is that it is not absorbed by dust and so can be used to study objects across our entire galaxy, even on the far side of our galaxy that would normally be obscured and inaccessible to us.

Part of the Galactic Plane of our galaxy seen by the ATLASGAL survey showing a number of prominent
star forming regions, including Messier 20, The Triffid Nebula (Credit: ESO/ATLASGAL)

The survey data has been used by a team of astronomers to survey the majority of our galaxy in the sub-mm part of the spectrum and identify hundreds of dense clumps of molecular gas where massive stars are forming. Sub-mm emission is one of the most reliable and efficient methods to identify dense star forming regions. It's an exciting project and I'm looking forward to seeing more results from the survey in the future.

The largest astronomical catalogs ever made! (Part II of II)

In a previous post I introduced the 'Top 10' largest astronomical catalogs that are currently and publicly available for astronomers (and anyone) to download and use. For a quick reminder, here's the list:

1. The US Naval Observatory (USNO-B1) all-sky catalog, 1046 million entries (2003)
2. The Guide Star Catalog (GSC v2.3) all-sky catalog, 946 million entries (2006)
3. The Sloan Digital Sky Survey (SDSS DR9), 933 million entries (2012)
4. The Wide-field Infrared Survey Explorer (WISE) all-sky catalog, 748 million entries (2013)
5. The United Kingdom Infrared Deep Sky Survey (UKIDSS), 727 million entries (2012)
6. The 2 Micron All Sky Survey (2MASS), 471 million entries (2003)
7. The Deep Near Infrared Survey (DENIS) of the southern sky, 355 million entries (2005)
8. The INT Photometric H-Alpha Survey (IPHAS), 219 million entries (2014)
9. The USNO CCD Astrograph Catalog v4 (UCAC4), 114 million entries (2012)
10. The Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE), 104 million entries (2008)

In that post I discussed five of these surveys (including the IPHAS survey that I work on - read more about that here) that all include astronomical objects observed in the visible part of the electromagnetic spectrum (that is the part that we can see with our own eyes). The other five of these surveys all include measurements made outside of this part of the spectrum, and its those I'd like to talk about today.

The electromagnetic spectrum, with wavelength increasing to the right, and frequency (or energy) increasing to the left. (Credit: NASA)

These five surveys are WISE, UKIDSS, 2MASS, DENIS, and GLIMPSE, and they're all infrared photometric surveys, which means they measure how bright astronomical sources are in the infrared. The infrared part of the electromagnetic spectrum includes radiation with a longer wavelength than that of the light we can see, though not as long as microwave radiation or radio waves. You may be aware that infrared radiation is what we commonly know of as heat or warmth. Any object with a temperature around body temperature or up to a few hundred Celsius will radiate energy in the infrared.

Astronomers often divide up the infrared part of the electromagnetic spectrum into the near-, mid- and far-infrared regions, ordered by how far they are from the visible part of the spectrum. The definition of these three regions isn't important, but originates in the different technologies required to detect radiation in each region (such as different CCD detectors).

The near-infrared is the most easily accessible part of the infrared spectrum, and three of these surveys cover this region. Perhaps the most commonly-used of these is 2MASS, an all-sky near-infrared survey, which operated from 1997 to 2001 at two observatories in Arizona and Chile. The 2MASS all-sky view of the Milky Way is a stunning panorama of our galaxy, aided by the ability of near-IR radiation to penetrate the dust that would otherwise absorb optically visible radiation.

The 2MASS all-sky view of the Milky Way (Credit: IPAC)

The DENIS survey was also a deep near-IR survey that finished in 2001, but which only covered the southern sky. The UKIDSS survey(s) are a series of very deep near-IR surveys (deeper than both 2MASS and DENIS) on the United Kingdom Infra-Red Telescope (UKIRT) in Hawaii that targeted certain areas of the sky for different science goals. Some of these areas include regions of our galaxy where stars are forming and other areas look away from our galaxy out into the darkest regions of space where other galaxies are easily studied.

The remaining two surveys, WISE and GLIMPSE, are mid-infrared surveys, operating at slightly longer wavelengths than the near-IR surveys.

The centre of our Milky Way galaxy seen in the mid-infrared from GLIMPSE (Credit: Spitzer GLIMPSE team)

GLIMPSE was a survey of the Milky Way galaxy with NASA's Spitzer Space Telescope, and WISE is also a NASA space telescope that performed an all-sky infrared survey. Because the Earth's atmosphere absorbs most mid-infrared radiation, mid-infrared telescopes have to operate from above the atmosphere.

An artist's impression of NASA's
Spitzer Space Telescope in orbit
(Credit: IPAC)

A further complication for mid-infrared telescopes is that the telescopes themselves have to be cooled to very low temperatures to prevent them from radiating infrared photons themselves! This is because, even in space objects will radiate infrared radiation depending on their temperature. These photons would contaminate the measurements made, and so the entire telescope has to be cooled to incredibly low temperatures, much lower than anything in our solar system, to prevent them from radiating significantly large amounts of infrared radiation. Both space telescopes used cryostats filled with liquid hydrogen to achieve this, allowing them to be cooled to less than -263 C (under 10 K).

That brings our list of the largest astronomical catalogs to a close, for the time being at least. Astronomy is moving ever so rapidly towards bigger and bigger catalogs and it is almost certain that these catalogs will be eclipsed in the near future. The most likely candidates to take the throne of largest astronomical catalog will probably come from either ESA's Gaia satellite or the Large Synoptic Survey Telescope (LSST). More on these in a future post!