Renaming multiples files with a bash loop

Question

I need to rename 45 files, and I don't want to do it one by one. These are the file names:

chr10.fasta         chr13_random.fasta  chr17.fasta         chr1.fasta          chr22_random.fasta  chr4_random.fasta  chr7_random.fasta  chrX.fasta
chr10_random.fasta  chr14.fasta         chr17_random.fasta  chr1_random.fasta   chr2.fasta          chr5.fasta         chr8.fasta         chrX_random.fasta
chr11.fasta         chr15.fasta         chr18.fasta         chr20.fasta         chr2_random.fasta   chr5_random.fasta  chr8_random.fasta  chrY.fasta
chr11_random.fasta  chr15_random.fasta  chr18_random.fasta  chr21.fasta         chr3.fasta          chr6.fasta         chr9.fasta         
chr12.fasta         chr16.fasta         chr19.fasta         chr21_random.fasta  chr3_random.fasta   chr6_random.fasta  chr9_random.fasta
chr13.fasta         chr16_random.fasta  chr19_random.fasta  chr22.fasta         chr4.fasta          chr7.fasta         chrM.fasta

I need to change the extension ".fasta" to ".fa". I'm trying to write a bash script to do it:

for i in $(ls chr*)

do

NEWNAME = `echo $i | sed 's/sta//g'`

mv $i $NEWNAME

done

But it doesn't work. Can you tell me why, or give another quick solution?

Thanks!