Vacuum Telescopes

The light from a celestial object is bounced and distorted as it penetrates the Earth’s turbulent atmosphere, and this image degradation continues all the way into the telescope.  Currents of air within the telescope tube caused by parts of the tube or optics being at different temperatures can severely degrade a telescope image, particularly in a large telescope.

Nowhere is this more apparent than in a professional solar telescope.  Sunlight entering the telescope heats up the inside of the telescope and optical components, resulting in turbulent air currents that make the images less sharp than they could be.

To solve this problem, some solar telescopes contain a vacuum so there is no air to heat and therefore no image distortion within the telescope.  This requires, however, a rather thick piece of glass (of high optical quality, of course) at the front of the telescope in order to maintain the vacuum within the tube.  A good example of this kind of telescope is the Swedish 1-m Solar Telescope (SST) located on the island of La Palma in the Canary Islands.

A much thinner front lens can be used if the telescope tube is filled with helium rather than evacuated, and though the results are much better than an air-filled telescope tube, they are not quite as good as with a vacuum telescope.

I am not aware of any vacuum telescopes being used for nighttime observations.

Meteor Watcher’s Network

I’ve been a meteor watching enthusiast since at least the early 1980s.  I had the good fortune back then of getting to know Paul Martsching when we both lived in Ames, Iowa, and few people in the world have logged more hours in the name of meteor science than he.  We have been close friends ever since.

We’ve learned that here in the U.S. Midwest, for any given astronomical event you wish to observe, there is between a 2/3 and 3/4 chance that it will be clouded out—unless you are willing to travel.  Weather forecasting has gotten much better over the years, and nowadays you can vastly improve your chances of not missing that important astronomical event, such as the Perseid meteor shower in August or the Geminid meteor shower in December.

Paul and I have traveled from Ames, Iowa to Nebraska, South Dakota, North Dakota, Kansas, Missouri, and Illinois over the years to escape cloudy skies.  Just last year, we had to travel to north of Jamestown, North Dakota to see the Perseids, and this year it appears we will need to travel to southern Kansas, Oklahoma, or Arkansas to get a clear view of the Geminids.

Weather forecasts don’t begin to get really accurate until about 48 hours out, so we often have to decide at nearly the last minute where to travel.  Therein lies the problem.  Where can we find a safe observing spot to put down our lawn chairs where there are no terrestrial lights visible brighter than the brightest stars, and no objectionable skyglow from sources or cities over the horizon?  It is a tall challenge.

What we need to develop is a nationwide network of folks who know of good places to watch meteors.  This would include astronomy clubs, individual astronomy enthusiasts, managers of parks and other natural areas, rural land owners who would allow meteor watchers on their land, rural B&Bs, cabins, lodges, ranches, and so on.  Once you know where you need to go to get out from under the clouds, there would be someone you could call in that area of the country to make expeditious observing arrangements for that night or the following night.  And perhaps lodging as well, if available.

If you would like to work with me to build a meteor watcher’s network or have ideas to share, please post comments here or contact me directly.

Do Dark Matter and Dark Energy Exist?

Numerous searches for the particle or particles responsible for dark matter have so far come up empty.  What if dark matter doesn’t really exist?  Could there be alternative explanation for the phenomena attributed to dark matter?

In the November 10, 2017 issue of the Astrophysical Journal, Swiss astronomer André Maeder presents an intriguing hypothesis that non-baryonic dark matter need not exist, nor dark energy either.  In “Dynamical Effects of the Scale Invariance of the Empty Space: The Fall of Dark Matter?” he suggests that scale invariance of empty space (i.e. very low density) over time could be causing the phenomena we attribute to dark matter and dark energy.

What is scale invariance?  In the cosmological context, it means that empty space and its properties do not change following an expansion or contraction.  Scales of length, time, mass, energy, and so on are defined by the presence of matter.  In the presence of matter, space is not scale invariant.  But take the matter away, and empty space may have some non-intuitive properties.  The expanding universe may require adding a small acceleration term that opposes the force of gravity.  In the earlier denser universe, this acceleration term was tiny in comparison to the rate at which the expansion was slowing down, but in the later emptier universe, the acceleration term dominates.  Sound like dark energy, doesn’t it?  But maybe it is an inherent property of empty space itself.

The existence of dark matter is primarily suggested by two  observed dynamical anomalies:

  1. Flat outer rotation curve of spiral galaxies (including the Milky Way)
  2. Motions of galaxies within galaxy clusters

Many spiral galaxies have a well-known property that  beyond a certain distance from their centers, their rotation rate (the orbital velocity of stars at that distance) stays nearly constant rather than decreasing as one would expect from Keplerian motion / Newtonian dynamics (think planets orbiting the Sun in our own solar system— the farther the planet is from the Sun, the slower it orbits).  Only there seems to be evidence that the rotation curves of galaxies when they are young (as seen in the high-redshift universe) do have a Keplerian gradient, but in the present-day universe the rotation curve is flat.  So, it appears, flat rotation curves could be an age effect.  In other words, in the outer regions of spiral galaxies, stars may be orbiting at the same velocity as they did in the past when they were closer to the galactic center.  Maeder writes:

…the relatively flat rotation curves of spiral galaxies is an age effect from the mechanical laws, which account for the scale invariant properties of the empty space at large scales.  These laws predict that the circular velocities remain the same, while a very low expansion rate not far from the Hubble rate progressively extends the outer layers, increasing the radius of the Galaxy and decreasing its surface density like 1/t.

We need to study the rotation curves (as a function of galactocentric radius all the way out to the outermost reaches of the galaxy) of many more galaxies at different redshifts (and thus ages) to help us test the validity of the scale invariant vs. dark matter hypotheses.  Maeder suggests a thorough rotation study of two massive and fast-rotating galaxies, UGC 2953 (a.k.a. IC 356; 50-68 Mly) and UGC 2487 (a.k.a. NGC 1167; 219-225 Mly), would be quite interesting.

The observed motions of galaxies within many galaxy clusters seems to indicate there is a substantial amount of unseen mass within these clusters, through application of the virial theorem.  However, the motions within some galaxy clusters such as Coma (336 Mly) and Abell 2029 (1.1 Gly) may be explainable without the need to resort to “exotic” dark matter.

Then there’s the AVR (Age-Velocity Dispersion Relation) problem which, incidentally, has nothing to do with dark matter.  But it may offer evidence for the scale invariant hypothesis.  It is convenient to specify the motion of a star in a spiral galaxy such as the Milky Way in a galactocentric coordinate system.

U = component of velocity towards the galaxy center

V = component of velocity in the direction of galactic rotation

W = component of velocity orthogonal to the galactic plane

Maeder writes:

The AVR problem is that of explaining why the velocity dispersion, in particular for the W-component, considerably increases with the age of the stars considered … Continuous processes, such as spiral waves, collisions with giant molecular clouds, etc… are active in the disk plane and may effectively influence the stellar velocity distributions.  However…vertical heating (the increase of the dispersion σW) is unexpected, since the stars spend most of their lifetime out of the galactic plane.

There may be more to “empty” space than meets the eye…

Maeder, A., 2017, ApJ, 849, 158

Theory and Observation

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

Thesis F1: Philosophical choices necessarily underly cosmological theory.
Some cosmologists tend to ignore the philosophical choices underlying their theories; but simplistic or unexamined philosophical standpoints are still philosophical standpoints!

Cosmology, and indeed all human inquiry, is based on (at least) two unproven (though certainly reasonable) assumptions:

  1. The Universe exists.
  2. The human mind is at least to some degree capable of perceiving and understanding the Universe.

Any cosmological theory will have additional foundational unproven assumptions.  These are called axioms.  Ellis admonishes us to at least be aware of them, and to admit to them.

8.1 Criteria for theories
As regards criteria for a good scientific theory, typical would be the following four areas of assessment: (1) Satisfactory structure: (a) internal consistency, (b) simplicity (Ockham’s razor), and (c) aesthetic appeal (‘beauty’ or ‘elegance’); (2) Intrinsic explanatory power: (a) logical tightness, (b) scope of the theory—the ability to unify otherwise separate phenomena, and (c) probability of the theory or model with respect to some well-defined measure; (3) Extrinsic explanatory power, or relatedness: (a) connectedness to the rest of science, (b) extendability—providing a basis for further development; (4) Observational and experimental support, in terms of (a) testability: the ability to make quantitative as well as qualitative predications that can be tested; and (b) confirmation: the extent to which the theory is supported by such tests as have been made.

As you can see, a theory is not an opinion.  It must be well-supported by facts.  It must be internally consistent.  It must have explanatory power.  The Russian physicist A. I. Kitaĭgorodskiĭ (1914-1985) put it succinctly: “A first-rate theory predicts; a second-rate theory forbids, and a
third-rate theory explains after the event.”  Einstein’s special and general relativity are spectacular examples of first-rate theories.  In over 100 years of increasingly rigorous and sophisticated experiments and observations, relativity has never been proven to be incorrect.

Ellis emphasizes the importance of observational and experimental support in any scientific theory.

It is particularly the latter that characterizes a scientific theory, in contrast to other types of theories claiming to explain features of the universe and why things happen as they do.  It should be noted that these criteria are philosophical in nature in that they themselves cannot be proven to be correct by any experiment.  Rather their choice is based on past experience combined with philosophical reflection.  One could attempt to formulate criteria for good criteria for scientific theories, but of course these too would need to be philosophically justified.  The enterprise will end in infinite regress unless it is ended at some stage by a simple acceptance of a specific set of criteria.

So, even our criteria about what makes a good scientific theory rest upon axioms that cannot be proven.  But unlike religion, scientific theories never posit the existence of any supernatural entity.

Thesis F3: Conflicts will inevitably arise in applying criteria for satisfactory cosmological theories.
The thrust of much recent development has been away from observational tests toward strongly theoretically based proposals, indeed sometimes almost discounting observational tests.  At present this is being corrected by a healthy move to detailed observational analysis of the consequences of the proposed theories, marking a maturity of the subject.  However because of all the limitations in terms of observations and testing, in the cosmological context we still have to rely heavily on other criteria, and some criteria that are important in most of science may not really make sense.

String theory? Cosmic inflation?  Multiverse? If a theory is currently neither testable nor directly supported by observations, is it science, or something else?

Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.

Average Orbital Distance

If a planet is orbiting the Sun with a semi-major axis, a, and orbital eccentricity, e, it is often stated that the average distance of the planet from the Sun is simply a.  This is only true for circular orbits (e = 0) where the planet maintains a constant distance from the Sun, and that distance is a.

Let’s imagine a hypothetical planet much like the Earth that has a perfectly circular orbit around the Sun with a = 1.0 AU and e = 0.  It is easy to see in this case that at all times, the planet will be exactly 1.0 AU from the Sun.

If, however, the planet orbits the Sun in an elliptical orbit at a = 1 AU and e > 0, we find that the planet orbits more slowly when it is farther from Sun than when it is nearer the Sun.  So, you’d expect to see the time-averaged average distance to be greater than 1.0 AU.  This is indeed the case.

The Earth’s current osculating orbital elements give us:

a = 0.999998 and e = 0.016694

Earth’s average distance from the Sun is thus:

Mercury, the innermost planet, has the most eccentric orbit of all the major planets:

a = 0.387098 and e = 0.205638

Mercury’s average distance from the Sun is thus:

Why are the Pleiades called the Seven Sisters?

The famous and beautiful Pleiades star cluster, which lies between 429 and 448 light years from us in the constellation Taurus the Bull, contains at least 2,109 stars that were formed around 125 million years ago—relatively recently on an astronomical timescale.  But when you look at the Pleiades with the unaided eye, unless you have unusually good vision and excellent sky conditions, you’ll see only six Pleiads.  If you see more than that, you’ll probably be able to see 8 or 9 Pleiads, maybe more.  But not seven.  So, why are the Pleiades called the Seven Sisters?

Here’s my conjecture.  Take a look at the Pleiades on a dark, moonless night. What do you see?  I think you’ll see a group of stars forming a tiny dipper shape, reminiscent of the much larger Little Dipper.

How many stars make up the Little Dipper shape?  Seven.  How many stars make up the Big Dipper shape?  Seven.  How many bright stars does nearby Orion have?  Seven.  Given this, and the fact that seven has long been considered a mystical number, it comes as no surprise, perhaps, that the Pleiades are called the Seven Sisters and not the Six Sisters or the Eight Sisters.  How many do you see?

The Pleiades will culminate1 at midnight for SW Wisconsinites on Friday night / Saturday morning, November 17/18.

1cross the celestial meridian; reach their highest point in the sky, due south

Bouy H., et al., 2015, A&A, 577, A148
Galli P. A. B., Moraux E., Bouy H., Bouvier J., et al., 2017, A&A, 598, A48
Stauffer J. R., Schultz G., Kirkpatrick J. D., 1998, ApJ, 499, L19

Changing Solar Distance

Between January 2 and 5 each year, the Earth reaches orbital perihelion, its closest distance to the Sun (0.983 AU).  Between July 3 and 6 each year, the Earth reaches orbital aphelion, its farthest distance from the Sun (1.017 AU).  These dates of perihelion and aphelion slowly shift across the calendar (always a half year apart) with a period between 22,000 and 26,000 years.

These distances can be easily derived knowing the semi-major axis (a) and orbital eccentricity (e) of the Earth’s orbit around the Sun, which are 1.000 and 0.017, respectively.

q = a (1-e) = 1.000 (1-0.017) = 0.983 AU

Q = a (1+e) = 1.000 (1+0.017) = 1.017 AU

So, the Earth is 0.034 AU closer to the Sun in early January than it is in early July.  This is about 5 million km or 3.1 million miles.

How great a distance is this, really?  The Moon in its orbit around the Earth is closer to the Sun around New Moon than it is around Full Moon.  Currently, this difference in distance ranges between 130,592 miles (April 2018) and 923,177 miles (October 2018).  Using the latter value, we see that the Moon’s maximum monthly range in distance from the Sun is 30% of the Earth’s range in distance from the Sun between perihelion and aphelion.

How about in terms of the diameter of the Sun?  The Sun’s diameter is 864,526 miles.  The Earth is just 3.6 Sun diameters closer to the Sun at perihelion than it is at aphelion.  Not much!  On average, the Earth is about 108 solar diameters distant from the Sun.

How about in terms of angular size?  When the Earth is at perihelion, the Sun exhibits an angular size of 29.7 arcminutes.  At aphelion, that angle is 28.7 arcminutes.

Can you see the difference?

Emergence of Complexity

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

7.3 Emergence of complexity
As the universe evolves an increase of complexity takes place in local systems as new kinds of objects come into being that did not exist before—nuclei, atoms, stars and galaxies, planets, life, consciousness, and products of the mind such as books and computers.  New kinds of physical states come into being at late times such as Bose-Einstein condensates, that plausibly cannot exist without the intervention of intelligent beings.

The first atoms formed about 400 thousand years after the Big Bang.  The first stars, at about 100 million years.  The emergence of atoms, stars, planets, life, intelligence, humans, morality, a Brahms symphony, etc. are a natural consequence of all the physical laws that existed at the moment of the Big Bang, 13.8 billion years ago.  There is nothing supernatural about the unfolding of the universe, remarkable as it is.  It is a completely natural process.  The only possibility of anything supernatural, I believe, is the cause of the Big Bang itself.  And, without scientific evidence…

We may never know or be able to understand the Big Bang, but the parturient possibilities contained in that creative moment are truly mind boggling: all that we see around us, all that was and is yet to be, existed then in a nascent state.  The universe as it evolves is not merely moving the furniture around, but it is creating entirely new structures and entities that never existed before.

Through the emergence of intelligence across billions of years, the universe has, at last, become self-aware.  Our consciousness is its consciousness.

Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.

Henry Norris Russell

Today, we celebrate the 140th anniversary of the birth of one of America’s greatest astrophysicists: Henry Norris Russell (1877-1957).  Called the “Dean of American Astronomers”, he is perhaps best remembered for his discovery of the relationship between the luminosity (absolute brightness) of a star and its color.  We call any plot of luminosity vs. color for a group of stars an H-R diagram, named after Russell and Danish astronomer Ejnar Hertzsprung (1873-1967) who independently discovered this relationship.

Russell noticed that cool (relative to other stars) red stars come in two varieties: those that are dim, and others that are very bright.  The only way a cool, red star could be so bright would be if the star were very, very large1.  In this way, Russell discovered that there are red giants and red dwarfs, but no medium-sized red stars.  Further studies by Russell and others led to the use of the H-R diagram as a tool in understanding the life cycles of stars.  Red giants, it turns out, are one of the final stages in the life of an ordinary star (like the Sun, for example).  Red dwarfs are low-mass stars that change very little throughout their lives.

After famously rejecting the revolutionary conclusion (in 1925) by Cecilia Payne-Gaposchkin (1900-1979) establishing that hydrogen is the primary constituent of the Sun and other stars, Henry Russell concluded four years later that Payne-Gaposchkin was correct, and acknowledged her significant contribution.  Moreover, he surmised that the main physical characteristics of stars are determined by just two basic parameters: mass and chemical composition.  This idea is known as the Vogt-Russell theorem, named after Russell and German astronomer Heinrich Vogt (1890-1968), who independently came up with the same idea.

An interesting sidenote.  Early in his stellar career, when he was just 24 years of age, Henry Russell wrote an interesting article published in the May 1902 issue of Popular Astronomy and dated March 24, 1902: “Shadows Cast by Starlight”.  It is a fascinating read—all the more special because it was written at a time (now over 115 years ago) when light pollution had not yet destroyed our nocturnal environment.

1Here we are comparing stars at comparable distances, such as in a star cluster.

ISS & SS Memories

The last Space Shuttle flight took place in July 2011 (Atlantis, STS-135), and in going through the archives from ten years ago, I found this write-up about the International Space Station and the Space Shuttle seen together in the sky.

International Space Station & Space Shuttle – Docked

This past Sunday evening brought my family to Governor Dodge State Park north of Dodgeville for a stroll in the dark—and what we thought would be a “routine” flyover of the International Space Station.  Boy, were we surprised!  Even though conditions were quite hazy, the ISS made its appearance as predicted, but as it reached its culmination of 62° at 10:27 p.m. (6/17/07 CDT) we witnessed something none of us had ever seen before: a gradual brightening of the ISS to between -6 and -9 magnitude, followed by a gradual dimming back to the normal slightly negative magnitude of a favorable flyover.  We had observed a “sun glint” off of the large station’s many reflective surfaces.  What a treat!

Footnote #1: The ISS had a definite orangish tint to us, which may have been real in spite of the hazy conditions.

Footnote #2: No-line bifocals (progressive lenses) work well during the day, but try looking at a bright moving object at night (or stars in general) to see just how bad the optics are!  For night viewing, I recommend a pair of glasses (if you need them) for distance viewing only, with glass lenses (not plastic!) and 0.5 diopter greater correction than you normally use.  I have such a pair, but forgot to bring them with me that night.

International Space Station & Space Shuttle – Undocked

This past Tuesday, the Space Shuttle Atlantis (STS-117) undocked from the International Space Station, and, as luck would have it, there were two opportunities that evening to view the pair—separated by only 46 miles—cross the sky in a beautiful pas de deux.  The first and best event, which culminated at 9:33 p.m. (6/19/07 CDT), was still impressive in spite of bright twilight because the spacecraft were so bright.  The brighter and oranger ISS was leading Space Shuttle Atlantis by about 3° when first sighted low in the NW, which expanded to about 6° at culmination since both spacecraft were closer to Wisconsin and the axis between the two least foreshortened, shrinking again to 3° when both spacecraft disappeared into the shadow of the Earth low in the ESE.  The changing orientation of the axis connecting the two spacecraft as they crossed the sky was interesting to observe.

A curious phenomenon that my wife, daughter, and I all noticed was that the positions of the two spacecraft with respect to each other seemed to “wiggle” a bit at times as they crossed the sky.  What a strange optical illusion, because obviously both spacecraft were moving smoothly relative to Earth and relative to each other!

I also observed the second pass that evening, which reached a maximum altitude of only 14° in the WSW sky before the pair entered the shadow of the Earth at 11:07 p.m. CDT.  Both spacecraft were about two magnitudes fainter than before, and this time Atlantis seemed brighter and oranger than the ISS!

As any double star observer knows, though, the perceived color of an object is strongly dependent upon its brightness!