What are some skywatching highlights you can see in July 2020? Enjoy the giant planets Jupiter and Saturn with their moons, stay up late to spot Mars rising. Plus: what would you see stargazing on the Red Planet? Additional information about topics covered in this episode of What’s Up, along with still images from the video, and the video transcript, are available at https://solarsystem.nasa.gov/whats-up….
In July, find the Scorpius constellation to identify the reddish supergiant Antares, which will lead you to discover a trio of globular star clusters. Keep watching for space-based views of these densely packed, spherical collections of ancient stars, as well as three nebulas: the Swan Nebula, the Lagoon Nebula, and the Trifid Nebula.
**What’s in the Night Sky July 2020 #WITNS | Lunar Eclipse | Milky Way | NLCs – Alyn Wallace
This illustration shows what the luminous blue variable star in the Kinman Dwarf galaxy could have looked like before its mysterious disappearance. Credits: ESO
Using the European Southern Observatory’s Very Large Telescope (VLT), astronomers have discovered the absence of an unstable massive star in a dwarf galaxy. Scientists think this could indicate that the star became less bright and partially obscured by dust. An alternative explanation is that the star collapsed into a black hole without producing a supernova.
“If true,” says team leader and PhD student Andrew Allan of Trinity College Dublin, Ireland, “this would be the first direct detection of such a monster star ending its life in this manner.”
Between 2001 and 2011, various teams of astronomers studied the mysterious massive star, located in the Kinman Dwarf galaxy, and their observations indicated it was in a late stage of its evolution. Allan and his collaborators in Ireland, Chile and the US wanted to find out more about how very massive stars end their lives, and the object in the Kinman Dwarf seemed like the perfect target. But when they pointed ESO’s VLT to the distant galaxy in 2019, they could no longer find the telltale signatures of the star.
“Instead, we were surprised to find out that the star had disappeared!” says Allan, who led a study of the star published today in Monthly Notices of the Royal Astronomical Society.
Located some 75 million light-years away in the constellation of Aquarius, the Kinman Dwarf galaxy is too far away for astronomers to see its individual stars, but they can detect the signatures of some of them. From 2001 to 2011, the light from the galaxy consistently showed evidence that it hosted a ‘luminous blue variable’ star some 2.5 million times brighter than the Sun. Stars of this type are unstable, showing occasional dramatic shifts in their spectra and brightness. Even with those shifts, luminous blue variables leave specific traces scientists can identify, but they were absent from the data the team collected in 2019, leaving them to wonder what had happened to the star.
“It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion,” says Allan.
Image of the Kinman Dwarf galaxy, also known as PHL 293B, taken with the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 in 2011, before the disappearance of the massive star. Located some 75 million light-years away, the galaxy is too far away for astronomers to clearly resolve its individual stars, but in observations done between 2001 and 2011, they detected the signatures of the massive star. These signatures were not present in more recent data. Credits: ESO
The group first turned the ESPRESSO instrument toward the star in August 2019, using the VLT’s four 8-metre telescopes simultaneously. But they were unable to find the signs that previously pointed to the presence of the luminous star. A few months later, the group tried the X-shooter instrument, also on ESO’s VLT, and again found no traces of the star.
“We may have detected one of the most massive stars of the local Universe going gently into the night,” says team-member Jose Groh, also of Trinity College Dublin. “Our discovery would not have been made without using the powerful ESO 8-metre telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO.” Ireland became an ESO member state in September 2018.
The team then turned to older data collected using X-shooter and the UVES instrument on ESO’s VLT, located in the Chilean Atacama Desert, and telescopes elsewhere.
“The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009,” says Andrea Mehner, a staff astronomer at ESO in Chile who participated in the study. “The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO’s newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view.”
The old data indicated that the star in the Kinman Dwarf could have been undergoing a strong outburst period that likely ended sometime after 2011. Luminous blue variable stars such as this one are prone to experiencing giant outbursts over the course of their life, causing the stars’ rate of mass loss to spike and their luminosity to increase dramatically.
Based on their observations and models, the astronomers have suggested two explanations for the star’s disappearance and lack of a supernova, related to this possible outburst. The outburst may have resulted in the luminous blue variable being transformed into a less luminous star, which could also be partly hidden by dust. Alternatively, the team says the star may have collapsed into a black hole, without producing a supernova explosion. This would be a rare event: our current understanding of how massive stars die points to most of them ending their lives in a supernova.
Future studies are needed to confirm what fate befell this star. Planned to begin operations in 2025, ESO’s Extremely Large Telescope (ELT) will be capable of resolving stars in distant galaxies such as the Kinman Dwarf, helping to solve cosmic mysteries such as this one.
[ Update June.12.2020: What’s Up: June 2020 – NASA JPL:
What are some skywatching highlights you can see in June 2020? Look for the Summer Triangle rising in the east after sundown, keep tabs on the morning planets and June 20 brings the solstice. Additional information about topics covered in this episode of What’s Up, along with still images from the video, and the video transcript, are available at https://solarsystem.nasa.gov/whats-up… Credit: NASA-JPL/Caltech
Though the nights are shorter in June, they are filled with fine sights. Look for the Hercules constellation, which will lead you to a globular star cluster with hundreds of thousands of densely packed stars. You can also spot Draco the dragon, which will point you to the Cat’s Eye Nebula. Keep watching for space-based views of globular star clusters and the nebula.
** What’s in the Night Sky June 2020#WITNS | Solar Eclipse | Milky Way | NLCs – Alyn Wallace
Astronomers using ESO telescopes have discovered giant spots on the surface of extremely hot stars hidden in stellar clusters, called extreme horizontal branch stars. This image shows an artist’s impression of what one of these stars, and its giant whitish spot, might look like. The spot is bright, takes up a quarter of the star’s surface and is caused by magnetic fields. As the star rotates, the spot on its surface comes and goes, causing visible changes in brightness.
Astronomers using European Southern Observatory (ESO) telescopes have discovered giant spots on the surface of extremely hot stars hidden in stellar clusters. Not only are these stars plagued by magnetic spots, some also experience superflare events, explosions of energy several million times more energetic than similar eruptions on the Sun. The findings, published today in Nature Astronomy, help astronomers better understand these puzzling stars and open doors to resolving other elusive mysteries of stellar astronomy.
The team, led by Yazan Momany from the INAF Astronomical Observatory of Padua in Italy, looked at a particular type of star known as extreme horizontal branch stars — objects with about half the mass of the Sun but four to five times hotter.
“These hot and small stars are special because we know they will bypass one of the final phases in the life of a typical star and will die prematurely,” says Momany, who was previously a staff astronomer at ESO’s Paranal Observatory in Chile. “In our Galaxy, these peculiar hot objects are generally associated with the presence of a close companion star.”
Spots on extreme horizontal branch stars (right) appear to be quite different from the dark sunspots on our own Sun (left), but both are caused by magnetic fields. The spots on these hot, extreme stars are brighter and hotter than the surrounding stellar surface, unlike on the Sun where we see spots as dark stains on the solar surface that are cooler than their surroundings. The spots on extreme horizontal branch stars are also significantly larger than sunspots, covering up to a quarter of the star’s surface. While sunspots vary in size, a typical size is around an Earth-size planet, 3000 smaller than a giant spot on an extreme horizontal branch star.
Surprisingly, however, the vast majority of these extreme horizontal branch stars, when observed in tightly packed stellar groups called globular clusters, do not appear to have companions. The team’s long-term monitoring of these stars, made with ESO telescopes, also revealed that there was something more to these mysterious objects. When looking at three different globular clusters, Momany and his colleagues found that many of the extreme horizontal branch stars within them showed regular changes in their brightness over the course of just a few days to several weeks.
“After eliminating all other scenarios, there was only one remaining possibility to explain their observed brightness variations,” concludes Simone Zaggia, a study co-author from the INAF Astronomical Observatory of Padua in Italy and a former ESO Fellow: “these stars must be plagued by spots!”
Spots on extreme horizontal branch stars appear to be quite different from the dark sunspots on our own Sun, but both are caused by magnetic fields. The spots on these hot, extreme stars are brighter and hotter than the surrounding stellar surface, unlike on the Sun where we see spots as dark stains on the solar surface that are cooler than their surroundings. The spots on extreme horizontal branch stars are also significantly larger than sunspots, covering up to a quarter of the star’s surface. These spots are incredibly persistent, lasting for decades, while individual sunspots are temporary, lasting only a few days to months. As the hot stars rotate, the spots on the surface come and go, causing the visible changes in brightness.
Beyond the variations in brightness due to spots, the team also discovered a couple of extreme horizontal branch stars that showed superflares — sudden explosions of energy and another signpost of the presence of a magnetic field.
“They are similar to the flares we see on our own Sun, but ten million times more energetic,” says study co-author Henri Boffin, an astronomer at ESO’s headquarters in Germany. “Such behaviour was certainly not expected and highlights the importance of magnetic fields in explaining the properties of these stars.”
After six decades of trying to understand extreme horizontal branch stars, astronomers now have a more complete picture of them. Moreover, this finding could help explain the origin of strong magnetic fields in many white dwarfs, objects that represent the final stage in the life of Sun-like stars and show similarities to extreme horizontal branch stars.
“The bigger picture though,” says team member, David Jones, a former ESO Fellow now at the Instituto de Astrofísica de Canarias, Spain, “is that changes in brightness of all hot stars — from young Sun-like stars to old extreme horizontal branch stars and long-dead white dwarfs — could all be connected. These objects can thus be understood as collectively suffering from magnetic spots on their surfaces.”
To arrive at this result, the astronomers used several instruments on ESO’s Very Large Telescope (VLT), including VIMOS, FLAMES and FORS2, as well as OmegaCAM attached to the VLT Survey Telescope at Paranal Observatory. They also employed ULTRACAM on the New Technology Telescope at ESO’s La Silla Observatory, also in Chile. The breakthrough came as the team observed the stars in the near-ultraviolet part of the spectrum, allowing them to reveal the hotter, extreme stars standing out bright amongst the cooler stars in globular clusters.
Red Giant stripped of mass during encounter with a black hole, leaving only a white dwarf in orbit. At closest point of its orbit, material from the white dwarf is pulled away by the black hole, resulting in X-ray pulses seen by Chandra X-Ray telescope. Credits: Chandra Observatory via Carnival of Space.
