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Chapter 24: Problem 18

Some galaxies in the Local Group exhibit blueshifted spectral lines. Why aren't these blueshifts violations of the Hubble law?

Short Answer

Expert verified
The blueshifts in the Local Group's galaxies do not violate Hubble's law because this law operates on a universal scale, while the blueshifts are local events. These blueshifts are caused by galaxies moving towards us, likely due to gravitational attractions within the Local Group, and do not contradict the overall universe expansion captured by Hubble's law.

Step by step solution

01

Understanding Blueshift and Redshift

Firstly, it is essential to know that the shift in spectral lines, either towards red or blue, signifies the movement of astronomical entities. A blueshift happens when an object is moving towards us, causing the wavelengths of light to decrease and shift towards the blue end of the spectrum. Conversely, a redshift happens when an object is moving away from us, causing the light's wavelengths to increase and shift towards the red end of the spectrum.
02

Comprehending Hubble's Law

Hubble's law posits that objects in the universe are generally moving away from us, the rate of which is directly proportional to their distance from us. Because most galaxies we observe exhibit a redshift, we can infer that they are moving away from us, affirming Hubble's law. This normally presents a paradox since a blueshift signals an object coming towards us.
03

Explaining Local Group Blueshifts

However, these blueshifts in the Local Group do not breach Hubble's law. This is because Hubble's law applies on a universal scale, it provides an overall snapshot of the universe's large-scale structure; it is not as applicable to local scales. In a local group where gravitational interactions between neighboring galaxies play a more substantial role, it is entirely possible and normal for galaxies to move towards each other, causing a blueshift in spectral lines.

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Most popular questions from this chapter

Why do astronomers suspect that the Hubble tuning fork diagram does not depict the evolutionary sequence of galaxies?

What is the Hubble classification scheme? Which category includes the largest galaxies? Which includes the smallest? Which category of galaxy is the most common?

Even though there are billions of galaxies, there are not billions of different kinds. In fact, galaxies are classified according to their appearance. As a group, dig into your book bags and put all of the writing implements (pens, pencils, highlighters, and so on) you have in a central pile. Remember which ones are yours! Determine a classification scheme that sorts the writing implements into at least three to six piles. Write down the scheme and the number of items in each pile. Ask the group next to you to use your scheme and sort your materials. Correct any ambiguities before submitting your classification scheme.

What is the Tully-Fisher relation? How is it used for measuring distances? Can it be used for galaxies of all kinds? Why or why not?

Why are Type Ia supernovae useful for finding the distances to very remote galaxies? Can they be used to find the distance to any galaxy you might choose? Explain.
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