In this new Hubble image, the strikingly luminous star AG Carinae — otherwise known as HD 94910 — takes centre stage. Found within the constellation of Carina in the southern sky, AG Carinae lies 20 000 light-years away, nestled in the Milky Way.

AG Carinae is classified as a Luminous Blue Variable. These rare objects are massive evolved stars that will one day become Wolf-Rayet Stars — a class of stars that are tens of thousands to several million times as luminous as the Sun. They have evolved from main sequence stars that were twenty times the mass of the Sun.

Stars like AG Carinae lose their mass at a phenomenal rate. This loss of mass is due to powerful stellar winds with speeds of up to 7 million km/hour. These powerful winds are also responsible for the shroud of material visible in this image. The winds exert enormous pressure on the clouds of interstellar material expelled by the star and force them into this shape.

Despite HD 94910’s intense luminosity, it is not visible with the naked eye as much of its output is in the ultraviolet.

This image was taken with the Wide Field and Planetary Camera 2 (WFPC2), that was installed on Hubble during the Shuttle mission STS-61 and was Hubble’s workhorse for many years. It is worth noting that the bright glare at the centre of the image is not the star itself. The star is tiny at this scale and hidden within the saturated region. The white cross is also not an astronomical phenomenon but rather an effect of the telescope.

Credit:
NASA/ESA and The Hubble Heritage Team (STScI/AURA)

[bottom left text]
Hubble Space telescope observed and identified the host star to a gravitationally lensed planet first discovered in 2003 by ground-based telescopes.

[left box]
A foreground red star and planet drifts toward the sky position of a much farther sunlike background star.

[middle-boxes]
In 2003, the foreground star-planet system slightly amplifies the light of a background star that momentarily aligns with it. This is called a microlensing event.

[right box]
The light from each star is progressively more offset year after year as the foreground star drifts by.

[bottom right box]
In 2005, Hubble Space Telescope observations distinguished the light from the two stars. This was possible because the foreground star turns out to be a different colour from the background star. By observing the stars though a red and blue filter, astronomers were able to enhance the visibility of the offset. The relative offset is 0.7 milliarcseconds (the angular width of a dime seen 3,000 miles away) from the source star. (This is below Hubble's resolution, but still a measurable effect.) The deduced positions of the two stars in 2005 are shown with red and blue crosshatches.

This post presents the results of HST observations of the host star for the first definitive extrasolar planet detected by microlensing.

The light curve model for this event predicts that the lens star should be separated from the source star by ~6mas at the time of the HST images.

If the lens star is a late G, K or early M dwarf, then it will be visible in the HST images as an additional source of light that is blended with the source image.

Unless the lens and source have exactly the same colors, its presence will also be revealed by a systematic shift between centroids of the source plus lens in different filter bands.


The HST data indicates both of these effects: the HST source that matches the position of the source star is 0.21 magnitudes brighter in the ACS/HRC-F814W filter than the microlensing model predicts, and there is an offset of ~0.7mas between the centroid of this source in the F814W and F435W filter bands.

We conclude the planetary host star has been detected in these HST images, and this identification of the lens star enables a complete solution of the lens system.

The lens parameters are determined with a Bayesian analysis, averaging over uncertainties in the measured parameters, interstellar extinction, and allowing for the possibility of a binary companion to the source star.


This yields a stellar mass of M_* = 0.63(+0.07/-0.09) M_solar and a planet mass of M_p = 2.6 (+0.8/-0.6) M_Jup at an orbital separation of 4.3 (+2.5/-0.8) AU.

Thus, the lens system resembles our own Solar System, with a planet of ~3 Jupiter-masses in a Jupiter-like orbit around a star of two-thirds of a Solar mass.

These conclusions can be tested with future HST images, which should reveal a broadening of the blended source-plus-lens point spread function due to the relative lens-source proper motion.



Gravitational Microlensing can be thought of as a version of strong gravitational lensing in which the image separation is too small to be resolved.

Multiple images are formed, but their typical separation - ∆Θ ≉ 2 ΘE - is far below the limiting resolution determined by observational constraints.

Given the dependence of the Einstein radius on lens mass and geometry, it is clear that microlensing will occur for sufficiently small masses and sufficiently distant lenses and sources.



In very general terms, microlensing deals with the lensing effects of compact objects in the mass range 10-6 = m/M⊙ ≤ 10⁶.

This translates into Einstein radii/angular separations of a milli-arcsecond or smaller for the two main distance regimes: "galactic" - lens/source distances of order 10 kpc, and "extragalactic/cosmological" - lens/source distances of order Gpc.

Credit:
NASA/ESA and The Hubble Heritage Team (STScI/AURA)

NASA/ESA Hubble Space Telescope has trained its razor-sharp eye on one of the universe's most stately and photogenic galaxies, the Sombrero galaxy, Messier 104 (M104). The galaxy's hallmark is a brilliant white, bulbous core encircled by the thick dust lanes comprising the spiral structure of the galaxy. As seen from Earth, the galaxy is tilted nearly edge-on. We view it from just six degrees north of its equatorial plane. This brilliant galaxy was named the Sombrero because of its resemblance to the broad rim and high-topped Mexican hat.

At a relatively bright magnitude of +8, M104 is just beyond the limit of naked-eye visibility and is easily seen through small telescopes. The Sombrero lies at the southern edge of the rich Virgo cluster of galaxies and is one of the most massive objects in that group, equivalent to 800 billion suns. The galaxy is 50,000 light-years across and is located 30 million light-years from Earth.

Credit:
NASA/ESA and The Hubble Heritage Team (STScI/AURA)

This is an artist's concept of the exoplanet WASP-12b. It is the hottest known planet in the Milky Way galaxy, and potentially the shortest lived. The planet is only 3.2 million kilometres from its sunlike parent star — a fraction of Earth's distance from the Sun. Gravitational tidal forces from the star stretch the planet into an egg shape. The planet is so hot that it has puffed up to the point where its outer atmosphere spills onto the star. An accretion bridge streams toward the star and material is smeared into a swirling disc. The planet may be completely devoured by the star in 10 million years. The planet is too far away for the Hubble Space Telescope to photograph, but this interpretation is based in part on analysis of Hubble spectroscopic and photometric data.

Credit:
NASA/ESA and The Hubble Heritage Team (STScI/AURA)