Some nebulae create their own light, which sometimes has a fluorescent appearance, while others only reflect the light from nearby stars. Nebulae are often seen around dying stars, and it's thought they may be a natural process that occurs when a star is nearing the end of its life. Nebulae resemble thin clouds that have less atoms per cubic inch than a cloud of smoke.
When there is not enough thyroid hormone, neither the heart nor the blood vessels can function normally. In hypothyroidism, the reduced level of thyroid hormone causes the heart muscle to pump less vigorously, and eventually to become weakened. In addition, the heart muscle cannot fully relax after each heart beat.
Problems can cause abnormal heart rhythms, called arrhythmias. There are many types of arrhythmia. During an arrhythmia, the heart can beat too fast, too slow, or it can stop beating. Sudden cardiac arrest (SCA) occurs when the heart develops an arrhythmia that causes it to stop beating. This is different than a heart attack, where the heart usually continues to beat but blood flow to the heart is blocked. There are many possible causes of SCA.