Measuring the Universe—How Astronomers Try

by | Sep 1, 2023 | 0 comments

stellar parallax diagram
Stellar parallax diagram.

The claims that modern-day astronomers make about the size of the universe seem almost unbelievable. To better understand the tremendous distances they’re describing, we could imagine shrinking everything down to 1 billion times smaller.

The Earth would be about the size of a marble, about ½ inch (1.25 cm) across. At this scale, the Moon would be 15 inches (38 cm) away. The Sun, being much farther away, would be about 490 feet (150 m) in the distance and about 51 feet (15.5 m) in diameter. The planet Jupiter would be 2,550 feet (777 m) from the Earth (almost ½ mile), and Pluto would lie close to 4 miles (6.4 km) away.

Next, we take a big jump to go to the nearest star, Proxima Centauri. It would be over 25,000 miles (40,000 km) away; that’s over three times the actual diameter of the Earth. The Andromeda galaxy, the nearest major galaxy to the Earth, would lie at 15 billion miles, or 160 times the distance from the Earth to the Sun. These figures are incomprehensible even though we’ve shrunk the size of the universe to 1 billion times smaller.

Can we have confidence in these measurements? Here’s how the distances are measured.

Astronomers use more than ten methods to determine distances in space. Certain methods are often cross-checked with others to verify accuracy, yielding remarkably similar results considering the inaccessibility of deep space. One of the most reliable methods is the trigonometric parallax method. To use this method, astronomers check the position of a star twice over a period of six months. During that six-month span, Earth’s orbit carries us to another point in space about 186 million miles away. That causes the nearby star’s apparent position to shift compared to the background objects in the universe.

Initially, Earth-based telescopes were used to find the parallax of about 1,000 nearby stars. It was difficult to go farther because of the blurring effects of Earth’s atmosphere. Then the Hipparcos telescope was launched into space, and, over a period of four years, from 1989 to 1993, it found the distance to over 100,000 stars. Then in 2013, the larger Gaia telescope was launched and started taking measurements that were 200 times more accurate. It was able to determine the distance relatively accurately to over one billion stars. It routinely measures the shifting of a star’s position so small that it’s equivalent to the width of a human hair at a distance of 1000 km (621 miles).

Spectroscopic parallax is a method that determines a star’s distance by analyzing its light to see what type of star it is, then checking its brightness levels and using that variation to see how far the star must be. Many star types have a standard actual brightness. If one of those stars have an apparent brightness that’s very low, it’s most likely far away. Compensation is made for intervening dust which reddens and dims the star.

Most galaxies are extremely far away, too far to obtain a measurement from a single star. In those cases, astronomers can wait until a type Ia supernova explosion happens. These are exploding stars that can blaze with the brightness of billions of suns—bright enough to measure accurately. These stars are part of a binary system; they build up to an explosion by drawing material from a companion star. They will explode when they reach a specific mass, creating a specific brightness. This allows us to infer the distance based on the apparent brightness.

Cepheid variables are stars that pulsate in brightness. They’ve been found to have a specific brightness level that is tied to the rate of their pulsing. Since the actual brightness can be known by observing the rate of pulsing, the apparent brightness can show its distance from us.

Astronomers can also look at the spectrum of light coming from whole galaxies and determine how far their absorption lines are shifted to the red end of the spectrum. This shifting happens when objects move away, similar to the doppler effect of sound shifting when the source is moving toward or away from us. A train blowing its horn as it passes by demonstrates an easy-to-detect shift in pitch. Since space is expanding, galaxies that are very far away are moving away very fast. The redshift of each galaxy’s light shows the speed of its recession, and its distance can be inferred from that.

Some of these measurements show distances of more than 13 billion light years or 78,000,000,000,000,000,000,000 miles! That’s mind-numbing, but greater than the greatness of all mysteries of creation is the greatness of God who created the mysteries.

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