Tag Archives: Milky Way

Cosmic Distance Ladder – Part 4 of 4

The final part of the Cosmic Distance Ladder series.

How I See It

On March 15,1929, Edwin Hubble presented a paper to the National Academy of Sciences. A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae stated that the 24 objects he studied were receding from Earth in a specific pattern. The farther ones were receding faster. In fact, the distance vs recessional velocity was a linear direct proportion. This finding has had profound consequences on our understanding of the nature of the universe, when it originated, how large it is, and what future course it will take.

This is the final part of the series Cosmic Distance Ladder. Here are links to part 1part 2, and part 3. As promised, the arguments and information will be presented as conceptually as possible without emphasis on the mathematical details. The goal of this series has been to assist the non-technically trained reader to understand more about how we…

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Cosmic Distance Ladder – Part 3

How I See It

Part 3 of the Cosmic Distance Ladder series is a little longer than the previous two. We will see which tools astronomers use to find the distances to objects much beyond our own home galaxy. There will be some discussion of supernovae and black holes.

The first part of this post advances up the distance ladder by telling the remarkable story of Henrietta Swan Leavitt. Her contributions to the study of Cepheid variable stars led to a method to know intergalactic distances millions of light years from our Milky Way. The previous Cosmic Distance Ladder – Part 2 discussed stellar parallax and main sequence fitting as methods to determine distances to objects within the confines of the Milky Way vicinity. If you wish to see parts 1 & 2, they are linked here and here.

The Harvard Calculators

Henrietta was the daughter of Congregational church minister George Roswell Leavitt and Henrietta Swan…

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Syzygy ✩ Supernovae and Black Holes

“conjunction or opposition of a heavenly body with the sun,” 1650s, from L.L. syzygia,
from Gk. syzygia “yoke, pair, union of two, conjunction,”
from syzygein “to yoke together,”
from syn- “together” + zygon “yoke” – Online Etymology Dictionary


Syzygy is one of my most favorite words. Astronomy and Space Exploration have many interesting connections. Stories often have several syzygies one can see.

What is a Supernova?
Stars which are several times more massive than our Sun end their lives in a spectacular explosion called a Supernova. The explosion occurs when the fuel for the fusion process in the core is depleted. This lack of outward pressure, which combats the inward gravitational pull, allows the star to collapse. As it shrinks, the core grows hotter and denser. New nuclear reactions begin and temporarily halt the collapse of the core. When the remaining core nuclear is basically just iron, nothing is left to fuse. Fusion in the core ends. Very quickly, the star begins its final gravitational collapse. The core temperature rises to many billions of degrees. The iron atoms are crushed together. The force of gravity is greater than the repulsive force between the nuclei of iron. The core then recoils. The energy of the recoil produces a shock wave through the star envelope. The envelope material is heated and fuses to form new and heavier elements and radioactive isotopes. The material is exploded away from the star core and is known as a supernova remnant. Many of these are seen. Here are examples.


The smaller supernovae leave behind a spinning neutron core only a few tens of miles across. Larger supernovae exert such tremendous inward shock forces that even the neutron core collapses into a black hole. It is so small and dense, that light is not fast enough to escape.

Turn Up Your Volume before you watch this video. It is an audio rendition of supernovae events in a small part of the sky. How it was done is explained below.

1. First, search for Supernovae over a long time interval.
From April, 2003 until August, 2006, the Canada-France-Hawaii Telescope (CFHT) watched four parts of the sky as often as possible. Armed with the largest digital camera in the known universe, CFHT monitored these four fields for a special type of Supernova (called Type Ia) which are created by the thermonuclear detonation of white-dwarf stars. These four fields covered roughly 16 times the area of the full Moon on the sky, or roughly 1/10,000 of the entire sky. Even though such a small fraction of the sky was monitored, 241 Type Ia Supernovae were seen during the period of observation.

The positions of all the Supernova are illustrated as time progresses. The animation is rendered at 15 frames per second, and each frame corresponds to just under a single day (1 sec of video = 2 wks of real time).

2. Assign each Supernova a note to play.
Distance to each Supernova determines the volume of the note. Closer is louder. Each Supernova follows a similar pattern of brightening and then fading. But they each also have some variations.

The pitch of the notes used was determined by the Supernova’s “stretch,” a property of how the Supernova brightens and fades. Higher stretch values played higher notes. The pitches were drawn from a Phrygian dominant scale for those who understand music theory.


3. Assign the instruments to be played.
Only two instruments were used. Notes of Supernovae in more massive galaxies were played by upright bass. Those in less massive galaxies were played by piano.

Creators of this Work – Alex H. Parker (University of Victoria) and Melissa L. Graham (University of California Santa Barbara / LCOGT).

Can Supernovae Cause a Black Hole?
Yes, they can. If a Supernova is more than several Solar masses, it can produce a Black Hole. You would not see the Black Hole. But, the influence of the Black Hole on the surroundings could be detected. Matter can be seen in a whirling accretion disc rushing around at tremendous speeds as it falls inward. There are often jets of high energy particles that are visible along the axis of spin. Light can be bent as it passes through the intense gravitational field from a distant galaxy to our telescopes. This gravitational lensing effect can distort images or cause multiple images of the distant galaxy.

Our own Milky Way galaxy apparently has a supermassive Black Hole at its core. It is about 4 million times the mass of our Sun. Detailed motion studies of the innermost stars reveal they are in orbit about an unseen object.

Click the Graphic to Watch Them Move


This animation region of space is exceedingly small at about 1×1 seconds of arc. It reveals the the innermost star paths from 1995 to 2010. The mass of the Black Hole (✩) is calculated from Kepler’s Laws of planetary motion. The arrow at the left side of the animation sets the distance scale at 0.1″ (seconds of arc). For reference, a circle = 360 degrees of arc. One degree = 60 minutes of arc. One minute = 60 seconds of arc. Hold your thumb at arms length and cover the Moon or Sun. That is 1/2 degree, or 30 minutes of arc. The Moon subtends 30×60 or 1800 seconds of arc. So, this patch of sky is tiny. It is about as large as the diameter of a medium sized crater on the Moon as viewed from Earth.

We are stardust.
If it weren’t for Supernovae, the heaviest elements would be iron. That is the top rung of the fusion process in star cores. Because of the tremendous shock waves of supernovae, fusions of  nucleii of elements heavier that iron are possible, giving us the much wider range of naturally occurring elements. Many of the elements in the rocks and minerals and our bodies came from a Supernova in our vicinity of space.