Close-orbiting binaries are a ticking time bomb. Over time they spiral ever closer to each other until they merge in a cataclysmic explosion such as a supernova. But in the middle of their story, things can get interesting. Some stars collapse into a white dwarf before merging with their partner, others edge so close to each other that their surfaces touch for a time, becoming contact binaries before finally colliding. But one newly discovered binary system will have a wild ride before its final demise.
The system is known as SSN 7, and it is a spectroscopic binary in the Small Magellanic Cloud. Spectroscopic means the two stars are so close to each other and so far away that we can’t resolve them as individual stars. Instead, we know they are binary by observing the redshift and blueshift of their spectral lines. From the spectral line data, astronomers can calculate their mutual orbits, and thus their masses.
The larger star is about 55 solar masses, and the smaller one is about 32 solar masses. Interestingly, the smaller one is the “primary” star, meaning that it’s the brighter of the two. This suggests that the smaller star is already feeding off the larger one in the early stage of merging. They orbit each other every three days, and their gravitational centers are only about 40 solar radii apart. They orbit so closely that they have to be a contact binary.
The mergers of compact objects discovered so far by LIGO and Virgo (in O1, O2 and O3a). The diagram shows black holes (blue), neutron stars (orange) and compact objects of unknown nature (grey), which were detected by their gravitational-wave emission. Each merger of a binary system corresponds to three compact objects shown: the two merging objects and the result of the merger. A selection of black holes (violet) and neutron stars (yellow) discovered by electromagnetic observations is shown for comparison. Image Credit: LIGO Virgo Collaboration / Frank Elavsky, Aaron Geller / Northwestern
Based on their orbits, the two stars will eventually merge in about 18 billion years. But given their masses, these stars won’t live long enough to merge. Stars above about 20 solar masses become supernovae before collapsing to become black holes. The larger one will likely become a black hole in about 700,000 years, and the smaller one about 200,000 years after that. This system will experience two supernovae within the next million years, only to merge as black holes billions of years later.
What makes this system particularly useful to astronomers is that it is squarely in the middle ground of binary systems. Most massive stars are part of close binary systems. We see lots of them as stable binaries not in the process of merging, and we have observed lots of merging stellar-mass black holes through gravitational wave astronomy. Until this system, we haven’t observed a merging binary system that will become a black hole merger. It gives us an excellent view of the elder years of these systems, which will help astronomers better understand their evolution.
Reference: Rickard, M. J., and D. Pauli. “A low-metallicity massive contact binary undergoing slow Case A mass transfer: A detailed spectroscopic and orbital analysis of SSN 7 in NGC 346 in the SMC.” arXiv preprint arXiv:2304.13720 (2023).
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Gravastars are an Alternative Theory to Black Holes. Here’s What They’d Look Like
One of the central predictions of general relativity is that in the end, gravity wins. Stars will fuse hydrogen into new elements to fight gravity and can oppose it for a time. Electrons and neutrons exert pressure to counter gravity, but their stability against that constant pull limits the amount of mass a white dwarf or neutron star can have. All of this can be countered by gathering more mass together. Beyond about 3 solar masses, give or take, gravity will overpower all other forces and collapse the mass into a black hole.
While black holes have a great deal of theoretical and observational evidence to prove their existence, the theory of black holes is not without issue. For one, general relativity predicts that the mass compresses to an infinitely dense singularity where the laws of physics break down. This singularity is shrouded by an event horizon, which serves as a point of no return for anything devoured by the black hole. Both of these are problematic, so there has been a long history of trying to find some alternative. Some mechanism that prevents singularities and event horizons from forming.
One alternative is a gravitational vacuum star or gravitational condensate star, commonly called a gravastar. It was first proposed in 2001, and takes advantage of the fact that most of the energy in the universe is not regular matter or even dark matter, but dark energy. Dark energy drives cosmic expansion, so perhaps it could oppose gravitational collapse in high densities.
Illustration of a hypothetical gravastar. Credit: Daniel Jampolski and Luciano Rezzolla, Goethe University Frankfurt
The original gravastar model proposed a kind of Bose-Einstein condensate of dark energy surrounded by a thin shell of regular matter. The internal condensate ensures that the gravastar has no singularity, while the dense shell of matter ensures that the gravastar appears similar to a black hole from the outside. Interesting idea, but there are two central problems. One is that the shell is unstable, particularly if the gravastar is rotating. There are ways to tweak things just so to make it stable, but such ideal conditions aren’t likely to occur in nature. The second problem is that gravitational wave observations of large body mergers confirm the standard black hole model. But a new gravastar model might solve some of those problems.
The new model essentially nests multiple gravastars together, somewhat like those nested Matryoshka dolls. Rather than a single shell enclosing exotic dark energy, the model has a layers of nested shells with dark energy between the layers. The authors refer to this model as a nestar, or nested gravastar. This alternative model makes the gravastar more stable, since the tension of dark energy is better balanced by the weight of the shells. The interior structure of the nestar also means that the gravitational waves of a nestar and black hole are more similar, meaning that technically their existence can’t be ruled out.
That said, even the authors note that there is no likely scenario that could produce nestars. They likely don’t exist, and it’s almost certain that what we observe as black holes are true black holes. But studies such as this one are great for testing the limits of general relativity. They help us understand what is possible within the framework of the theory, which in turn helps us better understand gravitational physics.
Reference: Jampolski, Daniel and Rezzolla, Luciano. “Nested solutions of gravitational condensate stars.” Classical and Quantum Gravity 41 (2024): 065014.
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Japan’s New H3 Rocket Successfully Blasts Off
Japan successfully tested its new flagship H3 rocket after an earlier version failed last year. The rocket lifted off from the Tanegashima Space Center on Saturday, February 17, reaching an orbital altitude of about 670 kilometers (420 miles). It deployed a set of micro-satellites and a dummy satellite designed to simulate a realistic payload.
With the successful launch of the H3, Japan will begin transitioning away from the previous H-2A rocket which has been in service since 2001 and is set to be retired after two more launches. Several upcoming missions depend on the H3, so this successful test was vital.
The launch came after two days of delays because of bad weather. The H3 rocket, built by Mitsubishi Heavy Industries, is now set to become the main launch vehicle of Japan’s space program. The rocket’s first flight in March 2023 failed to reach orbit, which resulted in the loss of an Earth imaging satellite.
The successful launch and deployment of the satellites was a relief for JAXA and members of the project. A livestream of the launch and subsequent successful orbit insertion showed those in the JAXA command cheering and hugging each other.
“I now feel a heavy load taken off my shoulders,” said JAXA H3 project manager Masashi Okada, speaking at a press briefing after the launch. “But now is the real start for H3, and we will work to steadily improve it.”
H3 stands about 57-meter (187-feet) tall and is designed to carry larger payloads. The two microsatellites were deployed approximately 16 minutes and 43 seconds after liftoff. They included an Earth observation satellite named CE-SAT-IE, developed by Canon Electronics, and TIRSAT, an infrared Earth observation instrument that will observe the temperature of the Earth’s surface and seawater.
“We feel so relieved to be able to announce the good results,” JAXA President Hiroshi Yamakawa said at the briefing. Yamakawa added that the main goals of H3 are to secure independent access to space and allow Japan to be competitive as international demand for satellite launches continues to grow. “We made a big first step today toward achieving that goal,” he said.
An image sent back by a mini-probe shows Japan’s SLIM lander on its side on the lunar surface. (JAXA / Takara Tomy / Sony Group / Doshisha Univ.)
The successful launch comes after two other recent successes for JAXA last month where the H-2A rocket successfully placed a spy satellite into orbit, and just days later JAXA’s robotic SLIM (Smart Lander for Investigating Moon) made the first-ever precise “pinpoint” Moon landing – although unfortunately the lander came down on its side. However, during the final stages of the descent two autonomous rovers were successfully deployed: a tiny hopping robot and the other designed to roll about the surface. Both have sent back pictures and can continue exploring and sending back information even if SLIM cannot be operated.
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European Satellite ERS-2 to Reenter Earth’s Atmosphere This Week
One of the largest reentries in recent years, ESA’s ERS-2 satellite is coming down this week.
After almost three decades in orbit, an early Earth-observation satellite is finally coming down this week. The European Space Agency’s (ESA) European Remote Sensing satellite ERS-2 is set to reenter the Earth’s atmosphere on or around Wednesday, February 21st.
Trail Blazing Mission
Launched atop an Ariane-4 rocket from the Kourou Space Center in French Guiana on April 21st, 1995, ERS-2 was one of ESA’s first Earth observation satellites. ERS-2 monitored land masses, oceans, rivers, vegetation and the polar regions of the Earth using visible light and ultraviolet sensors. The mission was on hand for several natural disasters, including the flood of the Elbe River across Germany in 2006. ERS-2 ceased operations in September 2011.
Anatomy of the reentry of ERS-2. ESA
ERS-2 was placed in a retrograde, Sun-synchronous low Earth orbit, inclined 98.5 degrees relative to the equator. This orbit is typical for Earth-observing and clandestine spy satellites, as it allows the mission to image key target sites at the same relative Sun angle, an attribute handy for image interpretation.
ERS-2 tracks and ice floe. ESA
The Last Days of ERS-2
Reentry predictions for the satellite are centered on February 21st at 00:19 Universal Time (UT)+/- 25 hours. As we get closer, expect that time to get refined. The mass of ERS-2 at launch (including fuel) was 2,516 kilograms. Expect most of the satellite to burn up on reentry.
The orbital path of ERS-2. Orbitron
For context, recent high profile reentries include the UARS satellite (6.5 tons, in 2011), and the massive Long March-5B booster that launched the core module for China’s Tiangong Space Station in late 2022 (weighing in at 23 tons).
ERS-2 in the clean room on Earth prior to launch. ESA
ESA passed its first space debris mitigation policy in 2008, 13 years after ERS-2 was launched. In 2011, ESA decided to passively reenter the satellite, and began a series of 66 deorbiting maneuvers to bring its orbit down from 785 kilometers to 573 kilometers. Its fuel drained and batteries exhausted, ERS-2 is now succumbing to the increased drag of the Earth’s atmosphere as we near the peak of the current solar cycle.
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