The most massive star known is R136a1. It is a Wolf-Rayet star with a mass about 265 times larger than the Sun. Such a star is more massive than the assumed upper limit for a traditional star, and it is not entirely clear how such large stars can form. The most popular view has been that such supermassive stars form when two large stars collide, but new observations cast doubt on that model. 

The largest known star is part of a cluster of stars known as R136, which is in the Large Magellanic Cloud. The cluster is only about 1.5 million years old, and has about 70 bright blue (O-type) stars. A recent ultraviolet survey of the cluster found 9 stars with masses greater than 100 solar masses. Given the cluster’s age, it’s extremely unlikely that 9 pairs of large stars would have merged within that time. The most massive stars in this cluster also have a bright emission line known as  He II λ1640, and since the largest stars in the cluster are also the brightest, this emission line is prominent when the spectrum of the cluster stars are averaged together. This emission line also appears in another young star cluster in the galaxy known as NGC 3125. This would support the idea that the R136 cluster isn’t some unusual fluke.

So young star clusters might form supermassive stars directly rather than through mergers. If that’s the case, we’ll have to develop new models to account for them.

Paper: Paul A. Crowther, et al. The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II λ1640 in young star clusters. MNRAS 458 (1): 624-659 (2016). doi: 10.1093/mnras/stw273

The post Mystery Of The Largest Stars appeared first on One Universe at a Time.

IRAS 04368+2557 is a protostar about 450 light years from us.  It is a particularly young protostar, at about 300,000 years.  Because of its age and proximity, it provides an excellent opportunity to study the early stages of stellar and planetary formation.

Structure of a protoplanetary disk. Credit Sakai et al.

Like most protostars, this one has a protoplanetary disk out of which planets are expected to form.  Surrounding that is a larger protostellar envelope.  Between the two is a transition zone.  Because of interactions within the surrounding envelope, material can be slowed so that it gradually falls inward toward the protoplanetary disk.  This can enrich the chemical composition of the disk.  Now a new paper in Nature has found that the transition zone can have a centrifuge effect that chemically filters material entering the protoplanetary disk.

The team looked at line spectra of both the disk and the surrounding envelope.  The found that while the surrounding envelope was dominated by hydrocarbons, the region of the transition zone had high concentrations of sulphur monoxide.  In other words the chemical composition of the two regions are dramatically different.

This is likely due to a centrifugal effect where some molecules can more easily penetrate the transition zone, while others have a more difficult time.   So it seems that early solar systems are not simply the product of the material surrounding a young star, but that they are more dominated by materials that can penetrate the transition zone.

Paper:  Sakai N, Sakai T, Hirota T, et al. Change in the chemical composition of infalling gas forming a disk around a protostar. Nature. (2014)

The post Centrifuge appeared first on One Universe at a Time.

Above is an image of a protoplanetary disk in the Orion nebula, taken from the Hubble telescope.  The interesting thing about this particular image is that it’s edge on, and it’s silhouetted against the nebula itself.  This means we have a good view to the dust and rock of the disk.  This means we can actually observe the dust beginning to coalesce into larger bodies, which should eventually lead to planets.

The discovery of protoplanetary disks such as this one agree with the nebular hypothesis, which posits that a star and planetary system form together.  The star collapses out of a nebula, and it forms an accretion disk around itself, out of which the planets form.

While an image such as this is useful in observing the early stages of planetary formation, the very conditions that make it easy for us to observe also make it more difficult for planets to form.  The Orion nebula is dominated by four very bright stars known as the Trapezium cluster.  Because the surrounding nebula is brightly illuminated by these stars, the protoplanetary disk is easy to observe.  But this means the disk itself is also being irradiated by the cluster, so much of the gas and dust of the disk may be driven away from the star before planets have a chance to form.  It may be the case that some Jupiter-size planets can form quickly before too much gas and dust is driven away, but it isn’t likely that a solar system like ours will form in this case.

The post When Worlds Collide appeared first on One Universe at a Time.