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Hidden pulsating stars found on the far side of the Trifid Nebula

The ESO's VISTA telescope has used the near-infrared spectrum to peer through the gaseous cloud of the Trifid Nebula at the hidden stars behind.

The VISTA telescope's new view of the Trifid Nebula. ESO/VVV consortium/D. Minniti

Space is far from empty. You can't just peer into the far reaches and see all the stars, reaching back into infinite space: there are planets and asteroids and satellites and planetesimals and asteroid fields and giant, lovely clouds of gas and dust that obscure the view: nebulas.

That doesn't mean we can't see past them; we just can't do so in the visible light spectrum. That's why space telescopes are equipped with ultraviolet and infrared sensors, which can "see" those wavelengths even through other objects.

One such telescope is the European Southern Observatory's VISTA at the Paranal Observatory in Chile. One of its current projects is the VVV survey -- VISTA Variables in the Via Lactea. In this survey, commenced in 2010, the telescope is sweeping an area of around 520 square degrees around the southern plane and central bulge of the Milky Way galaxy, in five near-infrared bands, in order to provide a more accurate map -- and to find new and previously hidden objects.

The newly released image above is a case in point. It shows, just to the right of the centre, a region known as the Trifid Nebula -- some 5,200 light-years from Earth, located in the constellation of Sagittarius -- an odd combination of three kinds of nebula: emission, that is, a nebula that gives off its own light in the visible spectrum from gas ionised by the heat of nearby stars; a reflection nebula, that is, a nebula that gives off no light of its own, but reflects the light of nearby stars; and a dark nebula, which gives off no light at all, but rather absorbs it.

The Trifid Nebula in its more recognisable form, rotated to the same orientation as the VISTA image. The reddish section is the emission nebula; blue is reflection; and the dark patches are the dark nebula.Adam Block, Mt. Lemmon SkyCenter, U. Arizona

When viewed in near-infrared, as seen in the top image, the nebula all but disappears, leaving behind just a few traces of the emission and dark nebulas -- and the stars behind are much more starkly visible. It is in this trove that the VISTA team found a pair of gems -- two Cepheid variable stars that appear quite close to the Nebula but that are a long way farther behind -- some 37,000 light-years from Earth. The heart of the Milky Way is 27,000 light-years distant.

Cepheid variable stars are stars that are unstable -- hence the "variable"; Cepheid is for one of the first Cepheid variable stars discovered, Delta Cephei in the constellation Cepheus, discovered by astronomer John Goodricke in 1784.

What this instability means in observable terms is that the star brightens, then dims, then brightens again, a long, slow pulse of variable light; in this case, that cycle is 11 days. This pair of stars, the team believes, are the brightest in a cluster, and are the only Cepheid variable stars discovered to date so close to the central plane on the far side of the galaxy.