The Pleiades

Map of the Pleiades Star Cluster

The Pleiades star cluster in Taurus (M45 in Messier’s catalogue) is about 135 million years old, which means any massive stars in the cluster would have exploded as supernovae at a time when ammonites thrived in the seas of Earth. According to the Danish physicist Henrik Svensmark, galactic cosmic rays from nearby supernovae strongly influenced the diversity of marine invertebrates by changing the Earth’s climate.

The cluster has hundreds of stars though only a few are visible to the unaided eye; the stars are thought to have formed together, and are 1/50^th the age of our Sun.

Venus approaching the Pleiades

The planet Venus approaching the Pleiades (or Seven Sisters) on 31^st March 2012, photographed by astronomy professor
Jimmy Westlake of Stagecoach, Colorado. A few days later the planet passed directly in front of the star cluster.

Hubble Space Telescope Refines Distance to the Pleiades

Astronomers using the Hubble Space Telescope have helped settle a mystery that has puzzled scientists concerning the exact distance to the Pleiades. There has been an ongoing controversy for the past seven years.

In 1997, the ESA’s satellite Hipparcos, the first space observatory to make precise measurements of the positions and motions of celestial objects, measured the distance to the Pleiades it found it is 10% closer to Earth than previous estimates. If the Hipparcos measurements were correct, then the stars in the Pleiades are peculiar because they are fainter than Sun-like stars would be at that distance. This finding, if substantiated, would challenge our basic understanding of the structure of stars.

But measurements made by the Hubble’s Fine Guidance Sensors show that the distance is about 440 light-years, essentially the same as past distance estimates and differing from the Hipparcos results by more than 40 light-years.

Recent measurements made at the California Institute of Technology and NASA’s Jet Propulsion Laboratory used interferometer measurements from Mt. Wilson and Palomar observatories, reporting that the star cluster is between 434 and 446 light-years from Earth.

The means for gauging distances to stars is the parallax method. With current telescopes, this method gives accurate results only for distances up to about 500 light-years. Distances beyond that are determined by indirect methods, based on comparing the brightness of distant stars with those of nearer ones of the same type, and assuming that both objects have the same intrinsic brightness.

According to David Soderblom, lead astronomer on the Hubble study, “The new Hubble result shows that the measurements made by Hipparcos contain a small, but significant, source of error that requires further exploration. New space missions are now being planned to carry out even more precise distance measurements out to greater distances”.

Kepler Sees Planet with Two Suns

NASA’s Kepler mission spotted a world where two suns set over the horizon instead of one. Nasa described the planet, called Kepler-16b, the most “Tatooine-like” yet discovered in our galaxy. Unlike Luke Skywalker’s home world Tatooine in Star Wars, the newly discovered planet is a cold world with a gaseous surface. But like Tatooine, it circles two stars. The larger, a K dwarf, is about 70% the mass of our sun, and the smaller, a red dwarf, is about 20% the sun’s mass.The system is in Cygnus. [An artist’s impression, not a photograph!]

Hot Young Stars

Hot stars being formed and shining brightly in blue light (Be careful if you type ‘Hot Young Stars’ into Google; you might get more than you expect!)

More Stars being Born in another Galaxy

[Right] Blue bursts of hot young stars in part of the spiral galaxy IC 5052 in Pavo. [Credit: Serge Meunier]

This image, speckled with blue, white, and yellow light, shows part of the spiral galaxy IC 5052. Surrounded by distant stars and galaxies, it emits a bright blue-white glow which highlights its narrow, intricate structure. It is viewed side-on in the constellation of Pavo (The Peacock), in the southern sky. When spiral galaxies are viewed from this angle, it is very difficult to fully understand their properties and how they are arranged. IC 5052 is actually a barred spiral galaxy – its pinwheeling arms do not begin from the centre point but are instead attached to either end of a straight “bar” of stars that cuts through the galaxy’s middle.

Approximately two-thirds of all spirals are barred, including the Milky Way. Bursts of pale blue light are visible across the galaxy’s length, partially blocked out by weaving lanes of darker gas and dust. These are pockets of extremely hot newborn stars. The bars present in spirals like IC 5052 are thought to help these formation processes by effectively funnelling material from the swirling arms inwards towards these hot stellar nurseries.

Vulpecula OB1, an area in our own galaxy where massive O- and B-type stars are forming

Star Formation in NGC 281

The star cluster NGC 281 is located about 9,200 light years from Earth in Cassiopeia and almost 1,000 light years above the plane of the Galaxy, giving astronomers an almost unfettered view of the star formation within it. This composite photograph contains x-ray data from the Chandra X-ray Observatory (purple) with infrared observations from the Spitzer Space Telescope (red, green, blue). The large columns of gas and dust on the left of the image probably contain newly forming stars. Emission from H, S, & O atoms in NGC 281 colours the nebula.

A Pulsar Racing Away

Astronomers found evidence of a pulsar (green) in the constellation Carina that is moving at 6 million miles per hour. Pulsars (pulsating stars) are rotating neutron stars that emit a beam of electromagnetic radiation that sweeps across the sky like a lighthouse beam. In purple are the remnants of a supernova explosion.

Wandering Star

Do you remember Lee Marvin’s song “I was born under a Wand’rin’ Star”? I was reminded of it when I saw this report of a star possibly with a planet that orbits it; they apparently came from another galaxy. The star is called HIP 13044 (and its suggested planet is HIP 13044b). See this account from The Guardian which includes a video simulation.

In 2010, it was announced that a giant planet in a 16.2-day orbit had been discovered by looking out for the small, gravitationally induced movements of its star as the planet orbits. The researchers measured the wobbles using a spectrograph connected to a 2.2-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile. This would have had implications for planet formation in metal-poor systems and survival of planets being engulfed by expanded giant stars. Subsequent analysis of the data revealed problems with the detection: for example an erroneous barycentric correction had been applied (the same error had also led to claims of planets around HIP 11952 that were subsequently refuted). After applying the corrections, there is no evidence for a planet orbiting the star.

HIP 13044 is a red horizontal-branch F-type star about 2,300 light years from Earth in the southern constellation Fornax. It is part of the “Helmi stream”, a former dwarf galaxy that merged with the Milky Way between six and nine billion years ago. As a result, HIP 13044 circles the galactic centre in a highly irregular orbit with respect to the galactic plane. HIP 13044 is slightly less massive than the Sun, but is approximately seven times its size. The star, estimated to be at least nine billion years old, has passed the red-giant phase. The relatively fast rotation of the star may be due to having engulfed one or more planets during the red-giant phase.

The Helmi stream is a stellar stream of the Milky Way galaxy. It was discovered in 1999, is formed of old stars deficient in heavy elements and has a mass of 10 to 100 million solar masses. It is a group of low-metallicity stars moving with large velocities relative to the Sun. The star follows an eccentric galactic orbit, with a distance from the galactic centre ranging from 7 to 16 kiloparsecs. The orbit does not lie in the galactic plane, and can reach distances as high as 13 kpc above it.

HIP 13044 is a fairly evolved star, fusing helium in its core, and has therefore already passed the red-giant phase of its evolution. It lies near the blue end of the red horizontal branch bordering the instability strip. Its surface temperature is about 6025 K and its radius is approximately 6.7 solar radii. HIP 13044’s mass is estimated to be 0.8 solar masses. Having a rotation period of 5 to 6 days, HIP 13044 is a fast-rotating star for its type. It is possible that this is because it has swallowed planets during its red-giant phase.

Fast Star First Fled from a Supernova, Now from the Galaxy

If a thermonuclear explosion went off next door, you’d run away too. The fastest known free-flying star in the galaxy is hightailing it out of here at 1,200 kilometres a second after surviving its sibling star’s death as a massive supernova. Clocking the star’s speed could pin down the nature of this stellar explosion and shed light on the cosmic ruler by which we measure the universe.

Known as US 708, the star was first discovered around 10 years ago and appears to be the remnant of a red giant that has been stripped of all its hydrogen, leaving behind a dense, hot core of helium that is around a third the mass of the sun.

Previous observations of the star revealed its radial velocity. Now Stephan Geier of the European Southern Observatory in Garching, Germany, and his colleagues have measured its full speed relative to the centre of the galaxy.

The team used the Keck Observatory on Mauna Kea in Hawaii to analyse the light from US 708, and found its full speed to be a little under 1,200 kilometres a second. They also discovered it was spinning quickly, at over 100 times a second, suggesting it received an energetic boost in its past.

Broken pair

All previously discovered hypervelocity stars – those going fast enough to escape the gravitational clutches of the Milky Way – are slower than US 708, and more similar in mass and temperature to our sun. These stars are thought to have originally been part of pairs that got too close to the supermassive black hole at the centre of our galaxy. One star got stuck orbiting the black hole, while the other was flung out at high speed.

But that explanation doesn’t work for US 708: something must have stolen its hydrogen before it sped up. This could only happen if it was in a very close pair with another star – so close that even a black hole couldn’t tear them apart.

Now Geier thinks he knows what happened. “The only way to get rid of the companion is a supernova,” he says. The idea is that US 708 was once in a bound pair with a white dwarf star that might have stolen all its hydrogen, leaving a helium core. The white dwarf, still not satisfied, then began sucking up helium until it became destabilised to the point of ignition, causing a massive supernova.

It was actually the breaking of the pair that propelled the helium star, not the explosion itself – a bit like when you hold hands with someone, spin in a circle and let go. “It’s not a shockwave, it’s the mere fact that this star is unleashed from this very tight binary,” says Geier. US 708’s fast spin also supports this explanation, he says.

If the theory is correct, finding more fast stars like US 708 could provide a way to study this kind of supernova. The scenario is a variant of an important process called a type Ia supernova. These are thought to always occur in one of a few ways, meaning the resulting explosions all have the same brightness.

Astronomers can use these “standard candles” to measure distances in the universe, but any variations in their origins could change the cosmic ruler, so it needs checking out. “If we can really prove this scenario, then we have a means to study those explosions in a new and very interesting way,” says Geier.

This article by Jacob Aron appeared in the New Scientist magazine on 5^th March 2015. Journal reference: Science, DOI: 10.1126/science.1259063

A Red Dwarf – A Star like Proxima Centauri

Proxima Centauri is a red dwarf. On the right is an artist’s suggestion of what star SO25300.5+165258 which is a red dwarf about 7.8 light years from the sun may look like.

Many factors appear to indicate that many red dwarfs, smaller than 30% of the Sun’s mass, have a very low probability for hosting indigenous life. Planets in the habitable zone of most red dwarfs would experience such a strong tidal heating that the hydrogen necessary for water and all known life would be ‘baked out’ of the planets before a stable orbit could be achieved, creating so-called ‘Tidal Venuses’. This star is in the constellation Aries.

Brown Dwarfs...

Brown dwarf stars: L-type...T-type...and the closest to us

...which are not really brown but are dwarf

Brown dwarfs are substellar objects too low in mass to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars, which can. They occupy the mass range between the heaviest gas giant planets and the lightest stars, with an upper limit around 75 to 80 Jupiter masses. Brown dwarfs heavier than about 13 Jupiter masses are thought to fuse deuterium (hydrogen-2) and those above about 65 Jupiter masses, fuse lithium as well. One debate is whether brown dwarfs are required to have experienced fusion at some point in their history.

Dwarfs are categorized by spectral classification (see the Hertzsprung–Russell Diagram), with the major types being M, L, T, and Y. Despite their name, most brown dwarfs would appear magenta to the human eye.

For some years now there has been debate concerning what criterion to use for defining the separation between a very-low-mass brown dwarf and a giant planet (about 13 Jupiter masses). One school of thought is based on formation, and another on interior physics.

Brown dwarfs may have fully convective surfaces and interiors, with no chemical differentiation by depth.

Some planets are known to orbit brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b.

The nearest known brown dwarf is Luhman 16 (WISE 1049-5319) a binary pair located in the southern constellation Vela at 6.6 light-years from the Sun. The WISE 1049-5319 binary pair of brown dwarf stars is the yellow disc at the centre of this 2010 WISE image. The individual brown dwarfs are not resolved.

One brown dwarf, DENIS-P J082303.1-491201 b, from an ultracool binary system, has a mass of about 28 Jupiter masses, making it the largest known exoplanet (as of March 2014).

High Energy X-Ray Star

Nasa’s Nuclear Spectroscopic Telescope Array (NuSTAR) (2012-031A) took its first snapshots of the highest-energy X-rays in the cosmos (lower insert, labelled ‘today’), producing images that are much sharper than those from previous high-energy telescopes (example in upper insert, ‘yesterday’). NuSTAR chose a black hole in the constellation Cygnus as its first target due to its brightness. Soon the mission will begin its exploration of hidden black holes, the remnants of supernova explosions and other sites of extreme physics. “Today, we obtained the first-ever focused images of the high-energy X-ray universe,” said Fiona Harrison, the mission’s principal investigator. “It’s like putting on a new pair of glasses and seeing aspects of the world around us clearly for the first time”. High-energy X-rays are produced as material falls into black holes – a final scream as they are swallowed up?

ζ (Zeta) Ophiuchi

This Spitzer Space Telescope infrared image obtained on 24^th December 2012 shows ζ Ophiuchi, a young, large and hot star around 370 light years away. It is about six times hotter, eight times wider, 20 times more massive and 80,000 times brighter than the Sun. The pink and blue whisps are of interstellar dust and have nothing to do with the star itself. [See chart.]

We are not Alone: Another Blue Planet

Scientists have discovered a second blue planet in the Universe, although this one is decidedly inhospitable and unlikely to support life.

Planet HD 189733b lies some 63 light years beyond our Solar System in the constellation Vulpecula and is a deep cobalt blue according to data gathered by the Hubble Space Telescope, but its azure hue is not due to water but drops of liquid glass raining down horizontally in 7,000 kilometre-per-hour winds.

By measuring the wavelengths of light that are lost when the orbiting planet slips behind its star, scientists have been able to calculate the colour of the planet as it would appear if seen by the naked eye.

It is the first time that scientists have been able to calculate the visible colour of an “exoplanet”, a planet beyond our own Solar System, said Frederic Pont of the University of Exeter, one of the authors of the study. “This planet has been studied well in the past, both by ourselves and other teams. But measuring its colour is a real first. We can actually imagine what this planet would look like if we were able to look at it directly,” Dr Pont said.

The planet is a gas giant, similar to Jupiter, and orbits very close to its star, meaning that its temperatures are a scorching 1,000°C or higher. Extreme winds pelt silicate particles sideways, which scatter blue light.

It was technically challenging to work out the colour of the planet because the light from its nearby star swamped any reflected light from the planet. However, by measuring the loss of light as the planet disappeared behind its sun, the scientists were able to assess the wavelengths that are reflected by HD 189733b. “We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star. From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured,” said Tom Evans of Oxford University, the lead author of the study.

Planets Forming round a New Star

The world’s highest radio telescope has captured an image providing evidence of how “gas” planets are formed. The telescope, built on a Chilean plateau in the Andes 5,000 metres above sea level, has captured the first image of a new planet being formed as it gobbles up the cosmic dust and gas surrounding a distant star.

Astronomers have long predicted that giant “gas” planets similar to Jupiter would form by collecting the dust and debris that forms around a young star. Now they have the first visual evidence to support the phenomenon, scientists said. The image taken by the Atacama Large Millimetre/submillimetre Array (Alma) in Chile shows two streams of gas connecting the inner and outer disks of cosmic material surrounding the star HD 142527, which is about 450 light-years from Earth in the constellation Lupus.

Astronomers believe the gas streamers are the result of two giant planets – too small to be visible in this image – exerting a gravitational pull on the cloud of surrounding dust and gas, causing the material to flow from the outer to inner stellar disks, said Simon Casassus of the University of Chile in Santiago.

“The most natural interpretation for the flows seen by ALMA is that the putative proto-planets are pulling streams of gas inward towards them that are channelled by their gravity. Much of the gas then overshoots the planets and continues inward to the portion of the disk close to the star, where it can eventually fall onto the star itself,” Dr Casassus said. “Astronomers have been predicting that these streams exist, but this is the first time we’ve been able to see them directly. Thanks to the new ALMA telescope, we’ve been able to get direct observations to illuminate current theories of how planets are formed,” he said.

The image, published in the journal Nature, appears to answer a long-standing conundrum of star formation: how does a new sun continue to grow by accumulating cosmic material when orbiting proto-planets are busy gobbling up the same source of cosmic dust and gas, creating huge gaps in the star-forming cloud of material. “This has been a bit of a mystery, but now we have found a process that allows the star to continue to grow despite the gap,” Dr Casassus said.

Vista Variables in the Via Lactea (VVV)

The Vista Variables in the Via Lactea (VVV) [‘Via Lactea’ means ‘Milky Way’] ESO Public Survey is an ongoing time-series, near-infrared (IR) survey of the Galactic bulge and an adjacent portion of the inner disk, covering 562 square degrees of the sky, using ESO’s VISTA telescope.

ESO reported the discovery of VVV BD001, a new member of the local volume-limited sample (within 20 parsecs from the sun) with well defined proper motion, distance, and luminosity. The spectral type of this new object is an L5±1, unusually blue dwarf. The proper motion for this BD is PM_α = −0.5455±0.004 sec/yr, PM_δ = −0.3255±0.004 sec/yr, and it has a parallax of 57±4 milliarcseconds which translates into a distance of 17.5±1.1 parsecs. VVV BD001 shows no evidence of variability (ΔKs <0.05magnitudes) over two years, especially constrained on a six month scale during the year 2012.

One of the results of these studies is discovery of “blue” L5 brown dwarf towards the Galactic centre in one of the most crowded regions of the sky. This object was previously observed by near-IR surveys (detection in 2MASS, DENIS and GLIMPSE) but have not been classified as a high proper motion object. VVV BD001 is not visible on SuperCOSMOS images. Spectral classification of VVV BD001 was made base on near-IR spectrum obtained with FIRE spectrograph at the 6.5 Magellan telescope at Las Campanas Observatory.

To obtain the parallax of the target, we used its equatorial coordinates on all available VVV images. Five bright, 13 to 14 magnitudes, and isolated stars without proper motion around the target were used to obtain the corrections to a common field center for each epoch. A sequence of 41 positions, used for estimation of the proper motion and the parallax, was obtained by averaging corrected coordinates date-by-date.

VVV BD001

VVV BD001 is a nearby brown dwarf of spectral type L5±1, located in constellation towards the Galactic bulge (Galactic Centre region) at approximately 57 light-years from Earth.

This new image, from ESO’s VISTA telescope, shows a newly-discovered brown dwarf nicknamed VVV BD001, which is located at the very centre of this zoomable image (inside the red circle). It is the first new brown dwarf spotted in our cosmic neighbourhood as part of the VVV Survey. VVV BD001 is located about 55 light-years away from us, towards the very crowded centre of our galaxy.

Brown dwarfs are stars that never quite managed to grow up into stars like our Sun. They are often referred to as “failed stars”; they are larger in size than planets like Jupiter, but smaller than stars.

This dwarf is peculiar in two ways; firstly, it is the first one found towards the centre of our Milky Way, one of the most crowded regions of the sky. Secondly, it belongs to an unusual class of stars known as “unusually blue brown dwarfs” – it is still unclear why these stars are bluer than expected.

Brown dwarfs are born in the same way as stars, but do not have enough mass to trigger the burning of hydrogen to become normal stars. Because of this they are much cooler and produce far less light, making them harder to find. Astronomers generally look for these objects using near and mid-infrared cameras and special telescopes that are sensitive to these very cool objects, but usually avoid looking in very crowded regions of space – such as the central region of our galaxy.

VISTA (the Visible and Infrared Survey Telescope for Astronomy) is the world’s largest survey telescope and is located at ESO’s Paranal Observatory in Chile. It is performing six separate surveys of the sky, and the VVV (VISTA Variables in the Via Lactea) survey is designed to catalogue a billion objects in the centre of our own Milky Way galaxy. VVV BD001 was discovered by chance during this survey.

Scientists have used the VVV catalogue to create a 3-dimensional map of the central bulge of the Milky Way. The data have also been used to create a monumental 108,200 by 81,500 pixel colour image containing nearly nine billion pixels, one of the biggest astronomical images ever produced.

A Star that should not Exist

This ancient star (called SDSS J102915+172927), in the constellation of Leo, should not exist according to astronomers’ current theories of star formation. It is too small and has too few elements heavier than hydrogen and helium to have condensed from clouds of gas and dust when it was born around 13 billion years ago and too old to have survived. And yet there it is in the picture...

Astronomy – The Stars

What is a Supernova?