A gigantic star makes off during an eight-year gap in observations.
- The massive star in the Kinsman Dwarf Galaxy seems to have disappeared between 2011 and 2019.
- It's likely that it erupted, but could it have collapsed into a black hole without a supernova?
- Maybe it's still there, but much less luminous and/or covered by dust.
A "very massive star" in the Kinman Dwarf galaxy caught the attention of astronomers in the early years of the 2000s: It seemed to be reaching a late-ish chapter in its life story and offered a rare chance to observe the death of a large star in a region low in metallicity. However, by the time scientists had the chance to turn the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Paranal, Chile back around to it in 2019 — it's not a slow-turner, just an in-demand device — it was utterly gone without a trace. But how?
The two leading theories about what happened are that either it's still there, still erupting its way through its death throes, with less luminosity and perhaps obscured by dust, or it just up and collapsed into a black hole without going through a supernova stage. "If true, this would be the first direct detection of such a monster star ending its life in this manner," says Andrew Allan of Trinity College Dublin, Ireland, leader of the observation team whose study is published in Monthly Notices of the Royal Astronomical Society.
Between astronomers' last look in 2011 and 2019 is a large enough interval of time for something to happen. Not that 2001 (when it was first observed) or 2019 have much meaning, since we're always watching the past out there and the Kinman Dwarf Galaxy is 75 million light years away. We often think of cosmic events as slow-moving phenomena because so often their follow-on effects are massive and unfold to us over time. But things happen just as fast big as small. The number of things that happened in the first 10 millionth of a trillionth of a trillionth of a trillionth of a second after the Big Bang, for example, is insane.
In any event, the Kinsman Dwarf Galaxy, or PHL 293B, is far way, too far for astronomers to directly observe its stars. Their presence can be inferred from spectroscopic signatures — specifically, PHL 293B between 2001 and 2011 consistently featured strong signatures of hydrogen that indicated the presence of a massive "luminous blue variable" (LBV) star about 2.5 times more brilliant than our Sun. Astronomers suspect that some very large stars may spend their final years as LBVs.
Though LBVs are known to experience radical shifts in spectra and brightness, they reliably leave specific traces that help confirm their ongoing presence. In 2019 the hydrogen signatures, and such traces, were gone. Allan says, "It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion."
The Kinsman Dwarf Galaxy, or PHL 293B, is one of the most metal-poor galaxies known. Explosive, massive, Wolf-Rayet stars are seldom seen in such environments — NASA refers to such stars as those that "live fast, die hard." Red supergiants are also rare to low Z environments. The now-missing star was looked to as a rare opportunity to observe a massive star's late stages in such an environment.
In August 2019, the team pointed the four eight-meter telescopes of ESO's ESPRESSO array simultaneously toward the LBV's former location: nothing. They also gave the VLT's X-shooter instrument a shot a few months later: also nothing.
Still pursuing the missing star, the scientists acquired access to older data for comparison to what they already felt they knew. "The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009," says Andrea Mehner, an ESO staff member who worked on 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."
Examination of this data suggested that the LBV may have indeed been winding up to a grand final sometime after 2011.
Team member Jose Groh, also of Trinity College, says "We may have detected one of the most massive stars of the local Universe going gently into the night. Our discovery would not have been made without using the powerful ESO 8-meter telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO."
Combining the 2019 data with contemporaneous Hubble Space Telescope (HST) imagery leaves the authors of the reports with the sense that "the LBV was in an eruptive state at least between 2001 and 2011, which then ended, and may have been followed by a collapse into a massive BH without the production of an SN. This scenario is consistent with the available HST and ground-based photometry."
A star collapsing into a black hole without a supernova would be a rare event, and that argues against the idea. The paper also notes that we may simply have missed the star's supernova during the eight-year observation gap.
LBVs are known to be highly unstable, so the star dropping to a state of less luminosity or producing a dust cover would be much more in the realm of expected behavior.
Says the paper: "A combination of a slightly reduced luminosity and a thick dusty shell could result in the star being obscured. While the lack of variability between the 2009 and 2019 near-infrared continuum from our X-shooter spectra eliminates the possibility of formation of hot dust (⪆1500 K), mid-infrared observations are necessary to rule out a slowly expanding cooler dust shell."
The authors of the report are pretty confident the star experienced a dramatic eruption after 2011. Beyond that, though:
"Based on our observations and models, we suggest that PHL 293B hosted an LBV with an eruption that ended sometime after 2011. This could have been followed by
(1) a surviving star or
(2) a collapse of the LBV to a BH [black hole] without the production of a bright SN, but possibly with a weak transient."
A young star and a belt of gasses give the game away.
- Scientists have provided the first confirmation that what's at the center of the Milky Way is a supermassive black hole.
- The discovery caught the interaction of gasses and a small star spinning around the mysterious object.
- This is thought to be compelling proof of the black hole's central role in a galaxy.
At the center of the Milky Way, about 25,000 light years away, is a faint source of radio noise. It's huge, estimated to weigh the equivalent of the 4.14 million suns. Astronomers have long suspected it's a supermassive black hole, and they've named it "Sagittarius A*." This week, the European Southern Observatory (ESO) announced that an international collaboration led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) has collected the most definitive proof that this is exactly what Sagittarius A* is.
While astronomers can't directly observe a black hole — light doesn't escape it — they might, however, be able to see some of what goes on around one. Genzel and other scientists across the globe collected information regarding a small star called "S2" and the belt of gas, or accretion disc, that spin around Sagittarius A*. It's in the interaction between the two that the new discovery lies, and it was made possible by a breakthrough in imaging.
The imaging breakthrough
Photo credit: MPE/GRAVITY team
The ESO has a four-telescope array, the Very Large Telescope (VLT) of the Paranal Observatory, rising 2635 meters above sea level in Chile's Atacama Desert. The amazing device that ultimately allowed the team to confirm Sagittarius A*'s identity leverages the Paranal telescopes. It's called "GRAVITY," and it combines all four in a single interferometer that has the resolution of a single mirror resolution of a single mirror 130 meters in diameter. "All of the sudden, we can see 1,000 times fainter than before," said Genzel when GRAVITY went into use.
S2 and the redshift
Image source: ESO/MPE/S. Gillessen
Every 16 years, a young blue star dubbed "S2" or S-02"completes an elongated orbit that brings it perilously close to Sagittarius A*, about 11 billion miles.
Many scientists feel that black holes — of which Einstein himself was unconvinced — are predicted by general relativity. (They were only finally confirmed a couple of years back when two black holes collided.) Einstein's theory, though, also predicts that if S2 is indeed orbiting a black hole, the speed of the light waves bouncing off it when it draws Sagittarius A*near should slow down, shifting the light it reflects to a more reddish hue.
In July of this year, Genzel's team announced that they had observed via GRAVITY the center of the Milky Way, and had seen the predicted redshift, allowing them to pinpoint S2's closest approach to Sagittarius A*. New York Times reports that as the results were being read off at the Munich announcement, the room broke out into applause.
Accretion disc flares
Also spinning around Sagittarius A* is an accretion disc that travels at nearly 30 percent the speed of light, zooming 150 million miles around the object every 45 minutes. According to relativity, whenever S2 — or any hot object — reaches its innermost, or stable, orbit, bits of it should cross the event horizon and be instantly vaporized as they fall into the black hole, sparking brief infrared flares.
Thanks to GRAVITY, the MPE scientists have been able to see that this actually happens at S2's closest fly-by. "GRAVITY's tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail," another MPR scientist, Oliver Pfuhl, tells ESO. "It's mind-boggling to actually witness material orbiting a massive black hole at 30 percent of the speed of light."
The predicted flares were spotted, actually, as the MPE team was observing S2 in the research that led to July's announcement, though it took until now to prepare supporting materials for publication. "We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*," Pfuhl recalls. "During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!"
Now we know what lies at the center of the Milky Way
Image source: ESO
Genzel refers to the discovery of the flares as a "resounding confirmation of the massive black hole paradigm." Astronomers believe that black holes likely lie at the core of other galaxies as well, so this announcement has far-reaching implications. "This always was one of our dream projects but we did not dare to hope that it would become possible so soon," he concludes.