Tag: astronomy

No Such Thing as Dark Matter

We’ve not been able to detect dark matter yet. Natalie Wolchover explains summarizes theories that could explain the way the universe works without having dark matter.

Key to it is the Modified Newtonian Dynamics (MOND) equation to explain why the stars at the outer edges of galaxies are moving faster than Newton’s force law predicts they should be.

Velocities of stars further away from the center of the galactic disk (larger R) have a higher velocity (V) than predicted by Newtonian physics. Dark matter has been used to explain this discrepancy, but tweaking the physics equations could do so as well. Image from Wikipedia.

Newton’s Second Law, finds that the Force (F) acting on an object is equal its mass (m) multiplied by its acceleration (a).

F = m \cdot a

The MOND equation adjusts this by adding in another multiplication factor (μ)

F = \mu \cdot m \cdot a

μ is just really close to 1 under “normal” everyday conditions, but gets bigger when accelerations are really, really small. Based on the evidence so far an equation for μ may be:

\mu = \frac{a}{a_0} \frac{1}{\sqrt{1+\left(\frac{a}{a_0}\right)^2}}

where, a₀ is a really, really small acceleration.

Factoring this μ factor into the equation for the force due to gravity ( F_g ) changes it from:

F_g = G \frac{ m_1 \cdot m_2}{r^2}

into:

F_g = G \frac{(m_1 \cdot m_2)}{r^2} + \frac{\sqrt{G \cdot \m_1 \cdot m_2 \cdot a_0}}{r}

The key point is that in the first term, which is our standard version, the denominator is the radius squared (r^2) while the second term has a plain radius denominator (r).

This means as the distance between two objects gets larger, the first term decreases much faster and the second term becomes more important.

As a result, the gravitational pull between, say a star at the edge of a galaxy and the center of the galaxy, is not as small as the standard gravitational equation would predict it would be, and the stars a the edge of galaxies move faster than they would be predicted to be without the additional term.

References:

What is Dark Matter?

Adam Hadhazy, in Discover Magazine, summarizes the top candidates to explain dark matter and the experiments in progress to find them. These include, WIMPs (Weakly Interacting Massive Particles, Axions, Sterile Neutrinos, and SIMPs (Strongly Interacting Massive Particles.

Distortions in the shapes of galaxies caused by gravitational lensing. While gravitational lensing is caused by anything with gravity (this means normal matter as well) the lensing effect of dark matter is a key form of evidence for its presence. Image of the galaxy cluster Abell 2218 via Wikimedia Commons.

via Brian Resnick on Vox, who provides some very interesting historical context on the discovery of dark matter.

Viewing the Night Sky with Stellarium

Jupiter shines above the moon. Image generated for St. Louis, MO, USA at 9:30pm on May 31, 2014 using the program Stellarium.
Jupiter shines above the Moon. Image generated for St. Louis, MO, USA at 9:30pm on May 31, 2014 using the program Stellarium.

I received an urgent email last night from a student who, while in the car last night, noticed a bright object above the moon. Was it a planet as her mom suggested? And if so which one? And do planets generate their own light or are we just seeing reflected light?

The last question was the easiest. The planets don’t generate light. You need something big and hot and fusiony, like a star, to generate light.

To figure out what the bright object was I did an internet search for star charts, and came across the Texas Astronomical Society’s webpage “Star Charts for Beginners“, which pointed me to the excellent, free program Stellarium.

You can use Stellarium to generate labeled images of the sky for almost any time, date, and place.

It looks like Jupiter could be seen above the Moon last night.

What Happens When Two Black Holes Collide?

A student asked this question about black holes during a discussion, and I didn’t have a good answer. Now there’s this:

A study last year found unusually high levels of the isotope carbon-14 in ancient rings of Japanese cedar trees and a corresponding spike in beryllium-10 in Antarctic ice.

The readings were traced back to a point in AD 774 or 775, suggesting that during that period the Earth was hit by an intense burst of radiation, but researchers were initially unable to determine its cause.

Now a separate team of astronomers have suggested it could have been due to the collision of two compact stellar remnants such as black holes, neutron stars or white dwarfs.

— via The Weather Channel (2013): Black Hole Collision May Have Irradiated Earth in 8th Century.

From the original article:

While long [Gamma Ray Bursts (GRBs)] are caused by the core collapse of a very massive star, short GRBs are explained by the merger of two compact objects … [such as] a neutron star with either a black hole becoming a more massive black hole, or with another neutron star becoming either a relatively massive stable neutron star or otherwise a black hole.

— Hambaryan and Neuhäuser (2013): A Galactic short gamma-ray burst as cause for the 14C peak in AD 774/5 in

More info via The Telegraph, and the original article discussing the spike in carbon-14 in tree rings is here.

Observing the Venus Transit

Shadow of the planet Venus during it's transit of the Sun on June 5th, 2012 at approximately 18:00 Central Time. Photograph taken from the MH Solar Observatory in St. Louis, MO, USA.

It’s pretty amazing how ecstatic seeing a simple circle with a little blobby dot can make a person feel. Following Ron Hipschman’s instructions, I installed a small aperture (~0.5 mm) solar projector at the newly established Muddle Home Solar Observatory (MHSO). The kids and I used it, and SunAeon’s app, to observe Venus transiting the Sun. It was, in a word, awesome.

The MHSO's small aperture (pinhole), solar projector.

For us the transit occurred late in the day, so by the end we had trees getting in the way.

Trees beginning to obscure the Sun.

If it seems odd that the trees are at the top of the image, it’s because the images in pinhole projectors are inverted. If I flip it around the right way, the image would actually look like this.

Corrected (inverted) image from the pinhole projector.

Painting the Universe: How Scientists Produce Color Images from the Hubble Space Telescope

The images taken by the Hubble Space Telescope are in black and white, but each image only captures a certain wavelength (color) of light.

The Guardian has an excellent video that explains how the images from the Hubble Space Telescope are created.

Each image from most research telescopes only capture certain, specific colors (wavelengths of light). One camera might only capture red light, another blue, and another green. These are captured in black and white, with black indicating no light and white the full intensity of light at that wavelength. Since red, blue and green are the primary colors, they can be mixed to compose the spectacular images of stars, galaxies, and the universe that NASA puts out every day.

Three galaxies. This image is a computer composite that combines the different individual colors taken by the telescope's cameras. Image from the Hubble Space Telescope via NASA.

The process looks something like this:

How images are assembled. Note that the original images don't have to be red, blue and green. They're often other wavelengths of light, like ultra-violet and infra-red, that are not visible to the eye but are common in space. So the images that you see from NASA are not necessarily what these things would look like if you could see them with the naked eye.

NASA’s image of the day is always worth a look.

How to Watch a Meteor Shower

A meteor shoots past the Milky Way. Image by L.Brumm Photography and Design.

Space.com has an excellent guide about the best way to observe meteor showers; dress warm; after midnight; be patient). The Lyrid meteor shower is on this week.

To take good photos of a shower you’ll need to do long exposures or get lucky. Details on the photo above here.