This is an artist's impression of innumerable Earth-like planets that have yet to be born over the next trillion years in the evolving universe.
This image from the NASA/ESA Hubble Space Telescope reveals a glistening and ancient globular cluster named NGC 3201 — a gathering of hundreds of thousands of stars bound together by gravity. NGC 3201 was discovered in 1826 by the Scottish astronomer James Dunlop, who described it as a “pretty large, pretty bright” object that becomes “rather irregular” towards its centre.
Globular clusters are found around all large galaxies, but their origin and role in galaxy formation remain tantalisingly unclear. Astronomers recently discovered a black hole lurking at the heart of NGC 3201 — its position was revealed by the strange movements of a star being quickly flung around a massive, invisible counterpart. This sparkling group of stars also has some strange properties which make it unique amongst the over 150 globular clusters belonging to the Milky Way. NGC 3201 has an extremely fast velocity with respect to the Sun and its orbit is retrograde, meaning that it moves speedily in the opposite direction to the galactic centre, which it orbits.
The unusual behaviour of this cluster suggests that it may have extragalactic origins, but at some point was captured by the Milky Way’s gravity. However, the chemical makeup of this intriguing cluster tells a different story — the stars within NGC 3201 are chemically very similar to those of other galactic globular clusters, implying that they formed at a similar location and time to their neighbours.
Whether this mysterious cluster was adopted by our galaxy or has for some reason evolved very differently to the family of clusters it grew up with, it is certainly an unusual astronomical beauty.
Acknowledgement: Sarajedini et al.
This is an image of one-half of the Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). The field contains approximately 150,000 stars, down to 30th magnitude. The stars in the Galactic disk and bulge have a mixture of colours and masses. The field is so crowded with stars because Hubble was looking across 26,000 light-years of space in the direction of the centre of our Galaxy.
Half of these stars are bright enough for Hubble to monitor for any small, brief and periodic dips in brightness caused by the passage of an extrasolar planet passing in front of the star, an event called a transit. Hubble took approximately 520 pictures of this field, at red and blue wavelengths, 22 to 29 February, 2004. The green circles identify 9 stars that are orbited by planets with periods of a few days. Planets so close to their stars with such short orbital periods are called 'hot Jupiters.'
These are considered 'candidate' extrasolar planets because most of them are too faint to allow for spectroscopic observations that would allow for a precise measure of the planet's mass. The Hubble observations allow for a robust statistical estimate of the possible 'false positives', which suggests that at least 45 percent of the candidates must be genuine planets.
The bottom frame identifies one of two stars in the field where astronomers were able to spectroscopically measure the star's back-and-forth wobble due to the pull of the planet. The planet turns out to be less than 3.8 Jupiter masses.
