How many stars are in our night sky? A lot more than we can see, that is for certain. And, according to a new map of the visible Milky Way -- the band of stars that represents the galactic disc visible from Earth -- There are at least 219 million. And that's just the part we can see.
The atlas is the culmination of more than a decade's worth of work by an international team of astronomers called the INT Photometric H-Alpha Survey (IPHAS), led by Professor Janet Drew at the University of Hertfordshire. According to the IPHAS website, the survey represents "all Galactic longitudes in the Northern Plane within the latitude range -5° < b < 5°, a total of 1800 square degrees".
The team used the Isaac Newton Telescope (INT) on La Palma in the Canary Islands. The Milky Way is so named because it appears as a clouded band in the sky -- and, as such, many of its stars are obscured from naked view -- presuming, of course, that we could see them all anyway.
Using the INT's 2.5-metre mirror, the team was able to chart all stars brighter than the 20th magnitude -- that is, 1 million times fainter than is visible to the naked human eye. The project took over 250,000 frames at a resolution of 8 megapixels each, with over 350 nights spent at the telescope taking the images.
This data was then compiled into a detailed map that shows the density of the stars -- a magnificent resource -- but, of even more value to astronomers, a massive catalogue, in which the properties of the stars are listed in table format.
"Constructing such a catalogue is a big data job, because it required the thousands of stars in each of the 250,000 CCD frames to be painstakingly measured. Needless to say, this required a high-performance supercomputer along with purpose-built software (for which the Cambridge Astronomical Survey Unit deserves much of the credit)," wrote the University of Hertfordshire's Dr Geert Barentsen, who worked on the project, in a blog post.
"The catalogue, which is 50 GB in size after data compression, is available for download from our project website and can be queried online through the Vizier data portal. In the near future, I will be blogging examples showing how the data may be accessed and exploited."
One of the most exciting features of the new data, Dr Barentsen said, was the use of a narrow-band filter that captured the brightest hydrogen emission line, called H-alpha. A very bright H-alpha signature indicates that a star is either very young or very old, surrounded by diffuse ionised gas -- a subclass of nebula. Because these stages of stellar evolution are short-lived, these stars can be hard to spot.
"Our catalogue will allow far more of these young and old objects to be discovered, which is necessary to develop and test models for these crucial stages in the life of a star. We have already identified several hundreds of previously unknown objects in the catalogue which are likely to be very young or old, and have started studying them in more detail," Dr Barentsen wrote.
In the nearly six-hour video below, you can get a glimpse of exactly how much data is being handled here: it shows all the images taken by the INT for the project, at a rate of one colour mosaic per second. You can also find more information about the project on the IPHAS website.