TGT #25F - Brown Dwarfs and X-rays and Triplets, Oh My!
TGT 4/1/25: This Just In--News About Brown Dwarf Stars and Archaeology of a Dead Star, A Hail and Farewell; Sky Gazing Calendar--A Time With No Moon and Planets, A Fast Re-emergence of Three Planets.
Cover Photo - A Triplet of Planets in the Dawn
In This Issue:
Cover Photo — A Triplet of Planets in the Dawn
Welcome to Issue #25F!
Sky Planning Calendar —
* Moon-Gazing - Moon Points You to the Pleiades, Jupiter and Mars
* Observing—Plan-et - Venus, Mercury and Saturn (Without Rings) Explosively Re-appear (Cover Photo); A Time With Only Stars
This Just In — Three Stories on Brown Dwarf Stars and One on a Dead Supernova
Welcome to The Galactic Times Newsletter-Inbox Magazine #25F!
Greetings, Galactic Timers!
Putting on my journalist hat this issue with a quartet of stories that are intriguing but not something covered by mainstream press. Find out more about Brown Dwarf stars, and doing archaeology of a dead star using X-rays in This Just In.
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Evening sky events? Not so much. The excitement is in the dawn where Mercury, Venus, and Saturn all zoom back into view. Zoom is the right word. They eject out of the solar glare by about a full degree per day, Venus going from barely visible 15-degrees from the sun to about 40 degrees!
But we also have a period of about a week where there are ONLY stars in the naked-eye sky—-no Moon, no planets—before dawn. Do some stargazing. Check out the Sky Planning Calendar.
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Enjoy!
Publisher — Dr. Larry Krumenaker Email: newsletter@thegalactictimes.com
Sky Planning Calendar
Moon-Gazing
Moon passages by a star, planet or deep sky object are a good way to find a planet or other object if you’ve never located it before.
April 1 The Moon passes by the Pleaides by just barely more than its lunar diameter, 0.6-degrees. At least the Seven Sisters don’t have to dive for cover as last month when the Moon passed over the star cluster.
April 2 Same ‘ol, same ‘ol’. Jupiter gets a wide berth from the Moon, the latter passing south of the former by 6-degrees.
April 4 First Quarter Moon.
April 5 Mars and Moon are a bit closer than Jupiter and Moon; the Moon passes just two-degrees north of the Red (but fading and shrinking and getting paler) Planet.
April 12 Full Moon and Apogee a half-day earlier. This makes this Full Moon the smallest for this year, although May’s will be a close second. While it is at it, it will pass a wide distance from the bright blue star Spica.
Observing---Plan-et
==Mercury, Venus, Saturn Rapidly Re-enter the Sky, in Dawn==
==A Planet-Free and Moon-Free Dark Sky Period in Early April==
Mercury and Venus dropped rapidly out of sight last month, and just as rapidly reappear in the dawn.
Mercury rises at least 45 minutes before the Sun starting on April 3rd, having been in solar conjunction only 10 days ago. But it doesn’t get much better than that for all of April and much of May, at least for Northern Hemisphereans. It is a much better apparition for those South of the Equator, the best morning apparition for the year there. You’ll find it 2-degrees to the left (East, up north) of Saturn on the 10th. (See the Cover Photo).
Venus mimics its little cousin. It was in solar conjunction 9 days before April 1st dawns and already it is rising more than an hour before the Sun. It, too, will look better in the Southern Hemisphere; up North it lies on the wrong side of the ecliptic and thus is closer to the horizon than it was the same number of days before the conjunction, by a long shot.
Both planets will get farther from the Sun, if not much farther from the horizon in Northern Hemisphere skies. By almost a degree a day farther. This will help Venus appear more easily and obviously. But it won’t appear in darkness before morning twilight for another month.
Speaking of Saturn, it TOO increases its distance from the Sun by almost a degree a day and around the 9th it rises at least 40 minutes before the Sun, hanging around with Mercury and getting as close as 2 degrees then. Once you find it, though, it will appear …. strange. You won’t see its rings. Earth just passed through its ring plane but we’re looking on its lower, southern side….which is NOT being lit by the sun. At best it is getting some reflected Saturn-light (Saturn-shine?) so the side we see is mostly dark! You MIGHT catch a glimpse of a very thin dark line crossing the visible disk of Saturn, that’s its shadow on the planet’s atmosphere. This Earth-on-one-side, Sun-on-the-other-side of the rings lasts for about a month, into early May, then Saturn gets its own autumnal equinox when the Sun goes from above Saturn’s northern hemisphere to its Southern.
Jupiter, sets around midnight Daylight Saving Time midmonth. It is your best evening planetary body—brightest, biggest in optics. But it is not in the morning sky for pre-dawn observers.
Mars follows Jupiter down three hours later so there is about a two-hour planet-free time in the night, between Martian setting and Venusian rising. So until April 5th, you’ll have DARK skies, weather permitting, with no Moon nor planets in this time period in the morning.
This Just In
It’s been a while since we visited the universe of scholarly journals. A few interesting notes that personify the incremental march of astronomy towards understanding the universe appeared in the past two weeks.
Brown Dwarfs
In The Astrophysical Journal Letters, Volume 981, Number 2, March 10, and in Volume 982 #1, two articles on Brown Dwarf stars caught my attention.
First, in “HCN and C2H2 in the Atmosphere of a T8.5+T9 Brown Dwarf Binary”, Elisabeth C. Matthews et al, DOI 10.3847/2041-8213/adb4ec, the researchers examined a particular T-dwarf. The coolest red fusion-burning stars are the M-dwarfs seen in the night sky. Cooler ones, not likely doing any fusion burning, extend the alphabet coding down to first L dwarfs and then T dwarfs. So why bother studying them at all?
Brown dwarfs are useful for studying the physics and chemistry of cold atmospheres. Although they may be glowing stars of a sort, their atmospheres are similar to those of many exoplanets. T-dwarf atmospheres are accessible by spectroscopic means, without all the complications due to light from a much brighter host star). As they cool, more molecules are found in their atmospheres, and need to be explained with increasingly complex chemistry.
T-type objects have strong water and ammonia absorption features in the mid-IR and in cool T dwarfs with high surface gravity, carbon is predicted to be primarily in the form of CH4, (methane). Also common should be NH3 (ammonia) and H2O. The James Webb Space Telescope is the first telescope in recent times that allows precise derivations of the molecular abundances, which in turn inform our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets.
The scientists in this article studied the brown dwarf with the not-rolling-off-your-tongue name of ‘WISE J045853.90+643451.9’. It appears to be cloud-free and has the usual three compounds mentioned above. But they also found the compounds HCN and C2H2 the poisonous Hydrogen Cyanide and the hydrocarbon fuel Acetylene, respectively.
How so? The cyanide suggests the atmosphere may be clear but also very windy, creating a lot of vertical mixing in the ‘air’ allowing these elements to separate from the simpler compounds and latching on to each other to form the more complicated ones. Similarly, perhaps, for the unexpected acetylene.
The mostly European authors conclude “This result challenges model assumptions about vertical mixing and/or our understanding of the C2H2 chemical network, or might hint towards more complex atmospheric processes such as magnetic fields driving aurorae or lightning driving ionization.”
Meanwhile, in the March 20th issue of the same journal, in Eccentricities of Close Stellar Binaries, Yanqin Wu et al, DOI 10.3847/2041-8213/adb751, say “Orbits of stellar binaries are in general eccentric. These eccentricities encode information about their early lives.” Using thousands of main-sequence binaries from the Gaia DR3 catalog reveals that binaries between 0 and a few astronomical units between companions form a Rayleigh distribution (like a normal bell curve with most of the left side cut off, a peak at some value and then a right-side tail off) It doesn’t matter if they measure “binaries from M to A spectral types, and/or from tens of days to thousands of days for an orbit. This commonality is likely primordial and its invariance suggests a single universal process.”
Ok, fine, so what?
The explanation, actually hypothesis to be tested, is that this happens because of weak scattering and ejection of brown-dwarf objects. This means that since at least half of all stars are actually binaries, the number of brown dwarfs in the galaxy might be at least the same number as the trillions of binaries in our Milky Way.
Speaking of binaries, a large part of astronomical statistical work is being done with Gaia data. The Gaia mission is recording and measuring the brightnesses and positions and often the spectra of every possible star in at least our side of the Milky Way, and also how they change over time. A lot of these are binaries. Whether the percentage is also near 50% of the stars isn’t stated in the article Orbital Inclination of Astrometric Binaries and the Dearth of Face-on Orbits in Gaia DR3 Solutions, by Valeri V. Makarov, in the same issue of ApJL as the Matthews article, points out that conclusion drawn regarding binaries have an inherent bias not found in visual binaries. We can see all the inclinations of the binary orbits from edge-on to face-on, but with Gaia data the face-on binary orbits don’t register. It is like looking for transiting planets across stellar disks and drawing generalizations when you are missing the planets that are just tilted enough in their orbits to miss the star’s disk and not show their existence. A statistical correction needs to be found.
Hail and Farewell, Gaia
Just noted…..the Gaia mission mentioned in above stories, after operating in Earth orbit for a decade, was shut down in January except for data transference and mechanical tests and matters. On this Thursday the 3rd, Gaia will fire its rocket booster onboard and be set free to orbit the Sun as its own little world, no data to be taken, no more communications. With over 2 billion stars recorded, plus galaxies and asteroids and some other objects found, it will be an astrometric database far beyond the work of all the astrometric astronomers ever lived. Would that all missions went to well.
Indiana Jones and the Former Exploded Star
Stellar archaeology. How do you dig into a star? You can’t. But if the star opens up its insides, you get an accounting for what that star was made of. That’s what getting data from NASA’s Chandra X-ray Observatory did, and several US and Israeli astronomers. They studied the relics one star left behind after it exploded. The star left its trail of debris around a binary with a 7-solar-mass black hole today and one star half as big. But the black hole used to be a star.
The system GRO J1655-40 has the former star’s debris striking its remaining star, but later some of that came falling back onto itself as the two objects distance shrunk with time until the black hole could pull gasses off the stellar companion, which include some of its own blown away stuff.
While most of this material sank into the black hole, a small amount of it fell into a disk that orbits around the black hole. Through the effects of powerful magnetic fields and friction in the disk, material is being sent out into interstellar space in the form of powerful winds.
Chandra detected signatures of individual elements found in the black hole’s winds with detailed spectra. The team reconstructed key physical characteristics of the star that exploded from the clues imprinted in the X-ray light by comparing the spectra with computer models of stars that explode as supernovae. They discovered that, based on the amounts of 18 different elements in the wind, the long-gone star destroyed in the supernova was about 25 times the mass of the Sun, and was much richer in elements heavier than helium in comparison with the Sun.
A paper describing these results titled Supernova Archaeology with X-Ray Binary Winds: The Case of GRO J1655−40 was published in The Astrophysical Journal in May 2024.