TGT #25K - 2 Million Stars Lead Us To A Better Milky Way - Outer Planets Pair with Brighter Ones
TGT 7/1/25: This Just In: Gaia and the Milky Way Galaxy; Sky Planning Calendar: Finding Uranus and Neptune the Easy Way.
Cover Photo - Farewell, Gaia
In This Issue:
Cover Photo — Farewell, Gaia
This Just In - Everything Gaia, from the European Astronomical Society Meeting in Cork, Ireland
Welcome to Issue #25K!
Sky Planning Calendar —
* Moon-Gazing - Sky Shows on the 4th (and 15th)
* Observing—Plan-et - Easy Ways to Find Uranus and Neptune
* NEW! - Calendar Summary
Welcome to The Galactic Times Newsletter-Inbox Magazine #25K!
Greetings, Galactic Timers!
Who says virtual meetings won’t give you jet lag? Five days ‘in’ Ireland, but online, is 3AM to noon Central Time. Oy….
Meanwhile, one of the largest number of presentations dealt with the late but not lamented Gaia mission from the European Space Agency. Launched in 2013 it was decommissioned after 10.5 years just this early spring. In this issue of TGT I will go over all the Milky Way discoveries this space observatory found with its nearly 2 million sources of light and billions of observations in the sky. A discussion of its future, the vast new data set coming, and for you who teach, some educational resources. Read in This Just In.
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The evening sky is getting leaner as Mars is fading and soon will enter the, um twilight zone. Saturn becomes the next evening planet though just barely. Dawn, like working in the sweltering heat, isn’t much to look forward to. However, if you want to see the ‘modern’ outer planets, i.e. Uranus and Neptune, this is your time. Using a pair of binoculars or a good telescope, Venus will show you Uranus nearby, just before morning twilight begins. Saturn (and for one day, the Moon) will get you even closer to Neptune, a mere degree, viewed from midnight onwards. Read about these in Sky Planning Calendar.
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Publisher — Dr. Larry Krumenaker Email: newsletter@thegalactictimes.com
This Just In
Everything Gaia, from the European Astronomical Society (EAS) Meeting in Cork, Ireland
The Gaia mission, created and operated by the European Space Agency (ESA), was launched in 2013. Once it arrived at the L2 Lagrangian point nearly a million miles outbound from Earth towards the edge of the solar system, it began rotating at a rate about one degree per minute, giving its two telescopes with auxiliary detectors to create in 10.5 years more of a cosmic census than much of the previous centuries of telescopic observation. Just under 2 million sources of light in the night sky, most of them stars in our Milky Way but also solar system objects like asteroids and galaxies beyond our own. Gaia recorded brightness, spectra, and motions (radial velocities towards and away from the Sun, and proper motions). Because there were many repeated observations of the sources, there is also a massive archive of time-domain date, variabilities of stars, changes in motions that indicate unseen companion objects, and more.
In this EAS meeting that covered more than six sessions with multiple speakers summarizing their work, it would take more pages than this issue of The Galactic Times has to describe all the reported research. Instead, this will be a summary of the results, from the past of Gaia to its present results on our home galaxy to Gaia’s future.
What future? Gaia is now an inactive probe, out of the gas it used for motion and stability. Its last science observation was in January 2025 but for two more months there were special calibrations and technology testing before it was fired up for the last time. Gaia’s final status is propelled into a heliocentric orbit, in which it will rendezvous with Earth every 14 years. On that last day of business, it was photographed from Earth:
But there is more to Gaia’s mission future than its mechanical parts. Here are several topics that had some extensive discussion.
The Data
The 3 trillion observations produced by Gaia’s two telescopes were released in batches. Called Data Release One, Two and Three (DR1, DR2 and DR3) , they were somewhat cumulative and improved in each round. The DRs allowed astronomers to work on their projects from almost the start (2016, 2018, 2020 and refreshed 2022, respectively). DR4 is expected to be released in December 2026, with 5.5 years of data and 500 Terabytes of information. It will contain time-series data, light curves of variable brightness objects (stars, quasars, asteroids), of radial velocity changes and changes in time of spectra, and a multitude of photos. The final full archive of data, DR5, is expected to be released no earlier than the end of 2030 and will contain every object and observation made during the probe’s lifetime of 10.5 years….in an ~1 Petabye database. Don’t try downloading that at home…..
The Milky Way Galaxy
Here are various reports on different aspects of galactic research.
One set of Gaia researchers analyzed stars within 1 kpc (one kiloparsec or about 3200 light years) to figure out some of the details in the Milky Way’s evolution, particular that of three main parts—its thin disk, its thick disk, and its halo. This was done in part by analyzing the metallicity of stars in the sample. Older stars, such as the Sun, have more metals in them because they are formed from the metal-rich debris of earlier stars that have exploded. The thin disk is like the Galaxy’s equatorial plane, inside the thicker disk which is up to a kiloparsec in thickness. What was a surprise is that the older stars are not found much in the thin disk; these stars are not much of a contributor to the thin disk. The thin disk is a place for stellar youngsters.
A contribution to the Solar System from this….if you were around 10 million years ago, you might have seen a really bright star in the night sky. Apparently a supernova occurred nearby, contributing high amounts of Beryllium to stellar spectra of stars near us of that age, and presumably us, too.
Meanwhile, the thick disk….what does Gaia tell us about the spiral arms of the Milky Way in it? There are four key facts to know. That disk is not flat as a pancake but it has perturbations in it. One such is the Galactic Warp, the outer precincts being curved upwards on one side, downward on the other. This is believed to be an aftereffect of a collision with a galaxy. In fact, it is just one such collision though this one has clearly left behind major structural changes.
Inside the arms and disk, though, there is not uniformity there either in this detail. The arms are not just thick or flat, and the stars follow the various small--scale variations. In fact, one might say the Galaxy is corrugated! Over-arching the general spiral pattern is a series of undulations, like ripples on a flat, non-moving pond surface. This relatively new discovery is called Radcliff Waves, noticed first in 2020. These move radially out of the Galactic Center. The Solar System passed through one (or it over us?) about 10 million years ago. Meanwhile the Local Arm and the Arms immediately beyond and inner to it, all show these undulations in profiles.
In the views below you can see the Radcliff Wave of the ‘recent’ past as a well-imaged near-linear structure. More informative perhaps is the side view, where the Z-axis distances above and below the general disk plane clearly show the undulations (the bottom chart).
The charts below show portions of those three arms (ours is in the middle) but the Y-axis view along the arm shows that the stars in the arm are carried away above and below with the Wave! If you look down the arm (the X-axis), i.e. doing a cross section of the arm, you see the stars are not filling the arm all over but are concentrated on the inner portion of the spiral arm.
But there is more about the motions of the arms, and how they come to be…or disappear.
For many years the dominant theory about how the spiral arms are made was the Density Wave theory. As this otherwise invisible wave of increased density rotating around the Galaxy, like a circular wave, hits material, like a bulldozer, the wave piles up the material in the area into a mass of nebulae and increased star formation. An alternative theory proposes that arms come and go because of more internal or regional dynamics, and the arms, or parts of them, get created or destroyed over time by the dynamical motions therein. The Gaia data seems to point more towards the Dynamical model as more pertinent. In fact, the two arms of the Milky Way farther out than the Sun, the Perseus Arm and then the prosaically-named Outer Arm, show their stars having opposite motions. The Perseus Arm’s are being disrupted, in fact moving out from the arm into the between-arm areas. The Outer Arm? The opposite. The stars are growing the Arm by moving inwards from both sides.
And last but not least, we have new and better maps of the Milky Way:
The Future
As noted at the start of this article, there is an impending Data Release, to be followed in a few more years with a Release of the ultimate archive of data. But mission scientists hope to see a new Gaia Mission launched, in 2050, the GaiaNIR. This is desired to explore beyond the local arms, the so-called hidden regions. The 2 million stars of today’s Gaia would be dwarfed by 12 billion stars of this new mission. Technologically, it would have (if approved) better spectroscopics in order to get more radial velocity measures. Further, because of the more than 20-year gap in observations, many stars can have their proper motions (the motion *across* the sky, not just towards and away) measured and thus their true total motions measured. It will observe with a wider range of wavelengths of light, particularly into the Near InfraRed, hence the new name.
Public Outreach
Some resources were promoted for educators and interested amateur astronomers.
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More material from the EAS meeting and the AAS two weeks before and the upcoming Royal Astronomical Society National Astronomy Meeting (in a week) coming in the next issue(s) of TGT.
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.
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July 2 First Quarter Moon.
July 3 The Moon passes South of the bright star Spica. It is also Earth’s Perihelion Day, when we are farthest from the Sun. See below.
July 4 A busy day and night! Apogee is today, smallest diameter Moon of the month, making the evening Moon a Micro(!)-Gibbous(fat!) Moon (isn’t THAT an oxymoron?). And with the Earth at Aphelion, max distance from the Sun, it is really far! Then there is a bunch of planetary stuff, see below.
July 6 The Moon occults (covers) a moderately bright and easy to see in binoculars star in Scorpius’ head, Pi Scorpii, before midnight for folks west of the Mississippi River (roughly) and after midnight on the 7th for folks farther East. Add or subtract an hour more or less for each time zone from the River you are.
July 7 In the dawn, following the occultation of Pi, the Moon passes less than its own diameter from the Scorpion’s Heart star, Antares.
July 10 Full Moon.
July 15 The Moon passes 3 degrees North of Uranus, and 4-degrees North of Saturn, almost simultaneously. Saturn will be the easier to see, though because of the lunar brightness binoculars will help even at that distance from the Moon.
Observing---Plan-et
==Evening Sky Gains Saturn, About To Lose Mars.==
==The Modern Planets (Uranus and Neptune) Can Be Found With Brighter Worlds==
Mercury turns the corner on its apparition on the 4th, reaching Greatest Eastern Elongation 26-degrees from the Sun in the evening twilight. But with it being amongst the Southern zodiac constellations, its daily positions are low and motions more parallel to the horizon for Northern Observers, but higher and more visible for those South of the Equator. Mercury begins afterwards to ‘reluctantly’ get more shy, setting sooner after the Sun each day (but always at least 40 minutes) and fading in brightness.
Catch it as the right endpoint of a lineup of three bright-ish evening objects on the 11th. From right to left (closest angularly to the Sun to farthest) Mercury, then Regulus, then Mars.
Also on the 4th, Venus passes Uranus by 2.4-degrees—Uranus will be down and left of the brilliant one of the pair, and in need of at least good binoculars to find it, though. It still though doesn’t rise up high, and only rises less than an hour before morning twilight begins. Getting smaller and ‘fuller’ as it begins to also turn a corner and head to a rendezvous with and behind the Sun.
Earth is as far from the Sun as it can get, on the 3rd, Aphelion Day. That may surprise many of you sweltering in summer heat, but not to the young girl in this YouTube clip that came up on my feed….bravo to her! https://www.facebook.com/share/r/1CE8UNgUhf/
Mars may not hit its solar conjunction until early next year but it is close to disappearing into the twilight. On the 1st it sets an hour after evening twilight, half that at midmonth. It might pass as a bright star in the western sky but not impressive at all, to the eye or a telescope. See under Mercury for a lineup with that planet and a star on the 11th.
Jupiter just passed behind the Sun and by the 7th it is visible rising into the morning twilight sky at least 40 minutes before Sunrise.
Saturn rises at Daylight Savings Time midnight on the 9th, following an hour and a few minutes of planet-less skies after Mars had set. It will be the only planet in the sky until Venus rises just before dawn. The slow-moving, barely-visible-with-thin-rings planet (and that not for long as they get nearly edgewise again later this year) hangs out with Neptune a degree away for weeks (they are technically closest on the 6th).
Calendar Summary
2 FIRST QUARTER MOON
3 Moon passes Spica; Earth reaches Aphelion, farthest from the Sun this year.
4 Moon reaches Apogee, farthest this month from Earth; Mercury reaches greatest elongation from the Sun, and Venus passes Uranus by 2.4°.
6 Moon occults Pi Scorpii (on the 7th for some); Saturn and Neptune technically closest today, less than 1° apart.
7 Moon passes 0.4° S of red giant Antares.
9 Saturn rises at midnight, earlier each subsequent day, becomes an evening planet.
10 FULL MOON.
11 Mercury is the right-most, Mars the left-most, of a line of three bright objects, —Mars—Regulus—Mercury in the evening twilight.
15 The Moon passes Saturn and Neptune almost simultaneously, by 4- and 3-degrees, respectively.